Adds a spec to verify extra header params on Client Credentials

This is a regression spec, based on the issue in #257 which
looks like it was fixed during a refactoring.

Author: nikz

spec/oauth2/strategy/client_credentials_spec.rb

@@ -10,6 +10,7 @@
         stub.post('/oauth/token', 'grant_type' => 'client_credentials') do |env|
           client_id, client_secret = Base64.decode64(env[:request_headers]['Authorization'].split(' ', 2)[1]).split(':', 2)
           client_id == 'abc' && client_secret == 'def' || raise(Faraday::Adapter::Test::Stubs::NotFound)
+          @last_headers = env[:request_headers]
           case @mode
           when 'formencoded'
             [200, {'Content-Type' => 'application/x-www-form-urlencoded'}, kvform_token]
@@ -66,4 +67,15 @@
       end
     end
   end
+
+  describe '#get_token (with extra header parameters)' do
+    before do
+      @mode = 'json'
+      @access = subject.get_token(:headers => {'X-Extra-Header' => 'wow'})
+    end
+
+    it 'sends the header correctly.' do
+      expect(@last_headers['X-Extra-Header']).to eq('wow')
+    end
+  end
 end