Fix custom object patch API content type in release-10.0 branch

[roycaihw]

Fix a regression in custom object patch API that can break existing clients who use JSON merge patch to patch custom objects.

ref: #866

NOTE:

1. the bug doesn't exist in master branch. It was introduced in Release 10.0.0 #864 (comment)
2. e2e test in master branch didn't exercise CRD and CR APIs. I added a test to demo the fix. See [DO NOT MERGE] Demonstrate custom object JSON merge patch failure #885 and [DO NOT MERGE] Demonstrate hotfix for #866 #886. We can merge that test into master branch later
3. we don't cherrypick the test into the release branch, since our travis-ci only works with the master branch.
4. long-term plan: Unsupported Media Type (HTTP 415) when patching custom resources #866 (comment)

/assign @yliaog
/kind bug
/priority critical-urgent

[yliaog]

/lgtm
/approve

[k8s-ci-robot]

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