REST API response header is text/html instead of application/json

When I activate the plugin the response I get for REST API requests are `text/html` instead of the default `application/json`. This breaks the parse of some packages, like superagent, that expect a json mime.

Reproduce the issue by activating the plugin and making a REST API request, then inspect the plugin's response handling to find where the Content-Type becomes text/html. Compare the result with the expected application/json header and verify the behavior with a package such as superagent.

Written by the indexing model from the issue text.