Is the space between Content-Length and "Content-Type: text/plain" yours or the program? Can you remove it manual and see what happens?

From what I recall of HTTP headers, any \n\n in the headers indicate the end of headers and the start of data, and without a content-type explicitly defined, it spits back that error.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
It's not what you know, but knowing how to find it if you don't know that's important