py2 http responses don't have a 'headers' attribute, so don't try to
use it.
Unfortunately the unit test infrastructure's FakeConn class does
have a 'headers' attribute (whose value doesn't match the result of
calling its 'getheaders()' method!), so this bug was not caught by
tests in the Related-Change.
Related-Change: I96f28ab0b2b5f9374c399e8905ee240e7b093f8b
Change-Id: I2cd820280b8c69cafc5730183903c9d379d8dde5
6e4ecfc5dc