Fix test for new WebOb
Fix test for new WebOb

In the change from WebOb 1.5.1 to 1.6.0, WebOb no longer adds a charset
when the content-type is application/json.

http://docs.webob.org/en/stable/whatsnew-1.6.html#bugfixes

Change-Id: I78431f295e891ddaf38809d7de55a7195fb9432d

gooddata/zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul