Change Content-Length to Int64

Change signature of various functions handling request and response
lengths from Int to Int64. Forces API version bump, now 0.6.0

Closes #27.
Merge branch '64-bit-length'
Test case for HTTP/1.0 lacking Content-Length

Make third fake HTTP response an HTTP/1.0 message, and remove its
Content-Length header to emulate an old server sending response body
signalling length by closing the connection.

Add failing test.
Handle HTTP requests without Content-Length

Ugly, but there are some HTTP servers which use neither Content-Length
nor chunked Transfer-Encoding, as was the case in HTTP/1.0 but is,
apparently, still permitted.

Closes #23.
Rename TestServer to MockServer

Better filename completion, since it is tests/TestSuite.hs that I'm
editing all the time.
Merge branch 'no-length'