Fix issue when http headers are already present

[Seldaek]

This fixes a regression introduced in #841

If a process/request runs:

• file_get_contents on an HTTP URL.
• then a json-schema validation using a file://... URL as schema (not all schema are remotely loaded)
• and http_get_last_response_headers() is available

Then you end up with:

• file_get_contents on the URL fills up the headers of that request
• json-schema validation loads the schema file but using file:// there are no headers, and it seems like file_get_contents does not clear previous headers itself
• http_get_last_response_headers() then returns the headers of the previous request
• and then $this->fetchContentType() will fail unless the response headers were of the correct application/schema+json mime type (which is very unlikely).

So this PR clears the headers before we fetch the schema, to make sure we have a clean slate. I'll probably report this to PHP as well because IMO it is a bit surprising.

[DannyvdSluijs]

I’noticed the same also. I’ll have a look this weekend or next week. Would that be ok, or do you need this fixed sooner?

If you could add an entry in the changelog that would help in getting it merged faster.

[Seldaek]

No rush at all, I've downgraded already :)

[Seldaek]

Thanks for wrapping it up for me