System tests have bitrotted.

[tseaver]

• Mangling os.environ in tests/system/requests/test_download.py is not 2.7-compatible. (PR Fix systest bitrot. #77)
• tests/system/requests/test_download.test_corrupt_download shows a mismash on the actual hash. (Issue Systest Failure: 'test_corrupt_download'. #76, xfailed in PR Fix systest bitrot. #77)
• test_chunked_download is getting an all-in-one response (inital response has no Location header, payload in body). (Issue Systest Failure: 'test_chunked_download'. #56, xfailed in PR Fix systest bitrot. #77)
• Default storage class is now MULTI_REGIONAL, not STANDARD, which breaks assertions in almost all the upload tests. No longer true: the default is STANDARD again.
• We need to set up the CI environment to run the system tests.(PR Fix systest bitrot. #77)

[tseaver]

The failure in test_corrupt_download occurs only for the gzipped file, which likely indicates that the test is exercising #50.