Fix status filtering

[jon-rtr]

From rundeck/rundeck#1923:

I've traced the issue down to the Content-Type header that messes up the whole request (It's not needed for a GET anyway, was there from a previous POST). The output is still a XML, I'll open a different issue for that

There are a few posts saying the same thing: GET requests specifying a Content-Type header causes the URL parameters to be ignored.