Fix incorrect content-type for help action

Author: azimux

src/http.rb

@@ -66,6 +66,7 @@ def request_to_command(request)
         command = super
 
         if request.action == "help"
+          request.set_response_header("content-type", "text/html")
           command.class.serializers = [Commands::Help::ResultSerializer]
         end
 

src/http/request.rb

@@ -109,6 +109,14 @@ def prefixless_path
                                path
                              end
         end
+
+        def set_response_header(name, value)
+          name = name.to_s
+          name = name.downcase
+
+          self.response_headers ||= {}
+          self.response_headers = response_headers.merge(name => value)
+        end
       end
     end
   end