InlineEdit: Test that non-ASCII characters aren't mangled

In I549037bfb BinaryResult was changed to return JSON string instead
of base64 encoded string, if the only value of the Accept request
header is application/json.

Adapt the unit tests to pass in Accept header to force REST endpoint to
return JSON instead of base64 encoded string. Add UTF-8 characters to
the content of commit message and file, to test that non-ASCII
characters aren't mangled.

Bug: issue 3093
Change-Id: Ida8a5cb9d486026d7fbe0c0bd46a6f8510cbe196
2 files changed