Exclude json content type from workhorse interception

What does this MR do?

This MR excludes application/json content type from workhorse interception when there is an error response. This is currently happening for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31669 - whenever user tries to update their username when username has been already taken, error response is in text/plain instead (generic 422 error page) of expected application/json

For json requests we would expect json response, without workhorse interception. There is no need in generic error pages in this case.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and Testing

N/A

Security

N/A