Fix copy-paste of JSON instead of XML in spec file

Similar to cb3d929

spec/unit/request_pattern_spec.rb

@@ -613,7 +613,7 @@ def match(request_signature)
             end
           end
 
-          context "standard application/json" do
+          context "standard application/xml" do
             let(:content_type) { 'application/xml' }
             it_behaves_like "a xml body"
           end