Extra boundaries are output when using BinaryStream/EncodedBinaryStream

[philbooth]

I see slightly different behaviour when using this crate with a BinaryStream than I do when using MultiPart<String>. I'm not sure if it is intentional, or maybe I'm doing something wrong.

To show what I mean, I tried to hack together a test case. If you apply this diff to current master:

diff --git a/Cargo.toml b/Cargo.toml
index afc418a..d0c219a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -27,3 +27,4 @@ futures = "^0.1"

 [dev-dependencies]
 tokio = "^0.1"
+tokio-core = "^0.1"
diff --git a/src/lib.rs b/src/lib.rs
index 655fcf1..6a88f67 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -462,6 +462,8 @@ extern crate textnonce;

 #[cfg(test)]
 extern crate tokio;
+#[cfg(test)]
+extern crate tokio_core;

 mod encoder;
 pub mod header;
diff --git a/src/mimebody.rs b/src/mimebody.rs
index c03123d..94a7bd4 100644
--- a/src/mimebody.rs
+++ b/src/mimebody.rs
@@ -501,7 +501,10 @@ where

 #[cfg(test)]
 mod test {
-    use super::{MultiPart, MultiPartKind, Part, SinglePart};
+    use futures::Stream;
+    use tokio_core::reactor::Core;
+
+    use super::{EncodedBinaryStream, MultiPart, MultiPartKind, Part, SinglePart};
     use header;

     #[test]
@@ -633,6 +636,22 @@ mod test {
                            "\r\n",
                            "<p>Текст <em>письма</em> в <a href=\"https://ru.wikipedia.org/wiki/Юникод\">уникоде</a><p>\r\n",
                            "--F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK--\r\n"));
+
+        let binary_part: MultiPart = MultiPart::new(MultiPartKind::Alternative)
+            .with_boundary("F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK")
+            .with_part(Part::Single(SinglePart::new()
+                             .with_header(header::ContentType("text/plain; charset=utf8".parse().unwrap()))
+                             .with_header(header::ContentTransferEncoding::Binary)
+                             .with_body(String::from("Текст письма в уникоде"))))
+            .with_singlepart(SinglePart::new()
+                             .with_header(header::ContentType("text/html; charset=utf8".parse().unwrap()))
+                             .with_header(header::ContentTransferEncoding::Binary)
+                             .with_body(String::from("<p>Текст <em>письма</em> в <a href=\"https://ru.wikipedia.org/wiki/Юникод\">уникоде</a><p>")));
+        let mut core = Core::new().unwrap();
+        let future = Into::<Box<EncodedBinaryStream<_>>>::into(binary_part).concat2();
+        let binary_part = String::from_utf8(core.run(future).unwrap().to_vec()).unwrap();
+
+        assert_eq!(part.to_string(), binary_part);
     }

     #[test]

I would expect the assertion assert_eq!(part.to_string(), binary_part); to succeed but it doesn't. Instead the second string contains extra boundary separators between the parts:

failures:

---- mimebody::test::multi_part_alternative stdout ----
thread 'mimebody::test::multi_part_alternative' panicked at 'assertion failed: `(left == right)`
  left: `"Content-Type: multipart/alternative; boundary=\"F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK\"\r\n\r\n--F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK\r\nContent-Type: text/plain; charset=utf8\r\nContent-Transfer-Encoding: binary\r\n\r\nТекст письма в уникоде\r\n--F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK\r\nContent-Type: text/html; charset=utf8\r\nContent-Transfer-Encoding: binary\r\n\r\n<p>Текст <em>письма</em> в <a href=\"https://ru.wikipedia.org/wiki/Юникод\">уникоде</a><p>\r\n--F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK--\r\n"`,
 right: `"Content-Type: multipart/alternative; boundary=\"F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK\"\r\n\r\n--F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK\r\n--F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK\r\nContent-Type: text/plain; charset=utf8\r\nContent-Transfer-Encoding: binary\r\n\r\nТекст письма в уникоде\r\n--F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK\r\nContent-Type: text/html; charset=utf8\r\nContent-Transfer-Encoding: binary\r\n\r\n<p>Текст <em>письма</em> в <a href=\"https://ru.wikipedia.org/wiki/Юникод\">уникоде</a><p>\r\n--F2mTKN843loAAAAA8porEdAjCKhArPxGeahYoZYSftse1GT/84tup+O0bs8eueVuAlMK--\r\n"`', src/mimebody.rs:654:9


failures:
    mimebody::test::multi_part_alternative

test result: FAILED. 44 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

Is this expected behaviour?