force the Content-Type to html in the profiler controller

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This just forces the Content-Type to match what will be returned, otherwise if the request format happens to be something else than HTML (which can be the case when building an app that only does JSON/XML with FOSRestBundle) it can happen that the Response class automatically sets a different Content-Type.

The approach taken here matches https://github.com/nelmio/NelmioApiDocBundle/blob/master/Controller/ApiDocController.php#L24