Fix brittle content-type check in ARes

This is for Jeremy’s(bitsweat) attention.

As Gabe pointed out in the comments, “This patch introduced a regression my app’s tests because our
activeresource mocks did not set a content-type header. This was
easy enough to fix once I debugged it, but it was pretty nasty to
track down since the result is simply that the error messages are
ignored and save returns true and the object appears valid with no
trace of what happened without debugging deep into ActiveResource.
Even though in production this case ought to never happen, in tests
it’s the default with no indication that you would need to set a
Content-Type header.”