[PHP] Correctly format JSON in headers

Created by: hinrik

ObjectSerializer::toHeaderValue() in the generated PHP code calls toString() on the values, which formats JSON with the JSON_PRETTY_PRINT option. This will result in a multi-line header which cannot be parsed since linebreaks aren't allowed by RFC 7230.

In my case I have a header schema called UpdateUser which I had hoped would be serialized as {"type":"staff","id":123}.

This PR adds a toHeaderValue() method to models which doesn't use JSON_PRETTY_PRINT. Works for me.

FYI @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @ybelenko @renepardon