Expose media endpoint only if RETURN_MEDIA_AS_URL is True

[tadejmagajna]

According to the documentation RETURN_MEDIA_AS_URL behaves as follows:

Set it to True to enable serving media files at a dedicated media endpoint. Defaults to False.

However, this is not entirely correct. Serving media files at a dedicated media endpoint is, in the current version of Eve, enabled regardless of what the value of RETURN_MEDIA_AS_URL is. This poses a serious security risk as most users that use RETURN_MEDIA_AS_BASE64_STRING = True won't be aware that their files can be served publicly via a url even if they explicitly set RETURN_MEDIA_AS_URL to False (it's False by default).

This change simply makes sure the media endpoint is enabled only if RETURN_MEDIA_AS_URL is True.

Side note: Current version of Eve does though allow us to disable the media endpoint by setting MEDIA_ENDPOINT to False, but this is an undocumented feature and users can only be made aware of it by browsing through this source code here. In order not to introduce too many breaking changes I decided to keep this code as is as it's unlikely to cause harm.

[nicolaiarocci]

rebased and merged, thank you!