Enforce current certificate retrieval behaviour

There appear to be no tests for retrieving certificates from the v2 api.

When you return a string from a webob request it inserts it as the body
in a standard WebOb Response. Because it returns a WebOb Response the
standard encoding behaviours in wsgi are not enforced and and all
responses are returned as straight text/html.

We can't change the behaviour of the certificate retrieval because it
will fail to be retrieved by auth_token middleware.

Change-Id: I8394f7ad2cc5a8f90b860170c2a5e3ecf334ac96
Merge "Enforce current certificate retrieval behaviour"