OAuth2 transport provider supporting of Legue OAuth Provider defaultHeaders()

[holdmann]

Some OAuth api-es require for default headers, that not currently passed to Curl-based requests. So adding support of this...

[TZK-]

Can you provide a real world API example that has this behavior please ?

I'm not sure we will merge this PR if it is to add few API Support.

But : It is totaly fine in that case to override the Transport (in your own class) and add your logic to handle your needs. I'm relatively perplex about the need to get this behavior in the core of php-api-wrapper.