Decode downloaded contents.

Also add a test (the exact url depends on pulling a change to pip-test-package).
Change test_requirements:test_remote_reqs to use hltbra's gist

g2p's pull request for issue #760 requires changes on pip-test-package.
The gist was created to remove this dependency.
Move python3/python2 details from ``download.get_file_content`` to backwardcompat module
Tweak variable name at backwardcompat.get_http_message_param (python 2 version)
handle unicode error in raw gzip data
more effective archive checking
only do content-type lookup on filename suspect links
move archive checks and introduce charset detection
filename is irrelevant when checking content-type
decode explicitly - don't use backcompat u() func
remove extraneous code path when setting charset
use backcompat get_param util for charset