Part 20: Fix URL encoding in JSON module WPT tests
We need to encode content_type by calling encodeURIComponent. Otherwise,
the plus sign is treated as a space.
Differential Revision: https://phabricator.services.mozilla.com/D218212
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1877792
gecko-commit: b0bf3998767e7d3dd70742809af2f41b48b553bd
gecko-reviewers: spidermonkey-reviewers, jonco