These 350 lines of Java code were inspired by Jacques Mattheij's The Several Million Dollar Bug. Please read the article to understand the background of the issue.
In Java technical details are as follows. Since HTTP protocol is synchronous by design (RFC 1945) NIO keeps OP_READ mode of the channel even OP_WRITE operation set on the first byte received. This is a standard implementation of non-blocking HTTP server you can find in Internet.
The magic goes on line #118. Comment it out and run the test.
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Requested: resources/tiny.gif
Received: 255
Received: 255
Received: 146
Transmitted: 43
...
In order to transmit 43 bytes we need to receive 656 bytes, huh. Now uncomment the line.
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Requested: resources/tiny.gif
Received: 255
Transmitted: 43
Received: 255
...
As we can see Java HTTP client accepts the forced response correctly. As well as the browsers. In your project make the request handler smarter by cutting off GET line only.
Have a nice day and several million dollar in a wallet!