Disable content_type checking in OFRAK Server testing code.

[whyitfor]

These changes avoid unhelpful content-type errors in OFRAK testing code. They do not impact test functionality.

• [ ] I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!) Update testing code to avoid unhelpful warnings on unexpected JSON content type.

Link to Related Issue(s) N/A.

Please describe the changes in your request. Testing code ignores these warnings to focus on actual test content.

Anyone you think should look at this, specifically? @EdwardLarson

[whyitfor]

@EdwardLarson, @dannyp303 I wonder if this change is actually needed, given #384?