fix gatekeeper response headers behavior

removes @wsgify decorator
changes code to a more traditional way of messing with response headers

Change-Id: I13ce7f98bae48eb0b990a64aa6f3d63ff8d13bf5
Closes-Bug: 1285392
Convert all old-style classes to new-style

This cleanup has been slowly happening for a while; let's finish it.

Change-Id: I1561e3540d524834e0cc5bc725ab80936eae1f0e
Merge "fix gatekeeper response headers behavior"