Add additionalProtocolParameters to protocolHeader
Add additionalProtocolParameters to protocolHeader

Merge additionalProtocolParameters with the baseProtocolHeaders in the
protocolHeader method. This appears to have been omitted, and is
required for Magento OAuth.

This also adds a doc comment for the fetchUserDetails method.

stevenmaguire/oauth1-client,thephpleague/oauth1-client,Gelembjuk/oauth1-client