make pypiserver compatible with pip 1.5

this basically removes the content-encoding header.

many thanks to @rmohr for reporting this issue in
https://github.com/schmir/pypiserver/pull/42:

,----
| it seems that since pip>=1.5, pip checks the response header of archives
| for Content-Encoding.
|
| This is what pypiserver currently guesses when you try to download a
| tar.gz archive:
|
| ('Content-Type', 'application/x-tar')
| ('Content-Encoding', 'gzip')
|
| What pip now does is decompressing the archive because of the
| Content-Encoding, without removing the .gz. Then it tries to untar
| it. Because of the filename it tries to decompress the tar archive
| again.
|
| This is what pypi returns:
|
| $ wget -S https://pypi.python.org/packages/source/p/pytz/pytz-2013.9.tar.gz
|   [...]
|   Content-Type: application/x-gzip
|   Accept-Ranges: bytes
|   [...]
|
| The patch below returns the same types as pypi does.
`----