Support attachments with empty data

[pandeyabs]

Attach returns an error here if an attachment file has no data. Is this by design? I don't see a reason why attachments can't be of size 0 bytes.

[meain]

diff --git a/attach.go b/attach.go
index 9aa48b6..809a8bb 100644
--- a/attach.go
+++ b/attach.go
@@ -108,7 +108,7 @@ func getAttachmentType(file *File) (attachType, error) {
 		return attachFile, nil
 	}
 
-	return 0, errors.New("empty attachment")
+	return attachData, nil
 }
 
 // attachB64 does the low level attaching of the files but decoding base64

FYI, this will be the only change we need (maybe add check for existence of filename?). With this change it still generates valid .eml files (I tried a bunch of online viewers and it was able to parse them without issues). I can submit a PR if you want (and probably add a toggle as folks might also want to be warned about attaching a empty file).

[xhit]

A toggle SupportEmptyAttachments should be added to avoid a breaking change. Feel free to send a PR. Thanks.

[meain]

I've created a PR at #98 . I've named the toggle as AllowEmptyAttachments instead of SupportEmptyAttachments as other toggles also star with Allow.