add unit tests for HTTPCompressHandler.

This adds unit tests for HTTPCompressHandler and its wrapper
HTTPCompressHandlerFunc. With these unit tests, a bug in the
GzipPool and the DeflatePool were identified where the data
written to a gzip.Writer resp. deflate.Writer wasn't completely
written, thus making decompression for the client side impossible
because some bytes were missing.
Merge pull request #2 from akrennmair/master

add unit tests for HTTPCompressHandler.