Ensure that the status summary is a string

Make sure that the content of `summary` can be searched for null bytes
and replaced with a valid XML string. Valid for the case of an XML
status response.

src/api/app/controllers/application_controller.rb

@@ -123,7 +123,7 @@ def required_parameters(*parameters)
 
   def gather_exception_defaults(opt)
     if opt[:message]
-      @summary = opt[:message]
+      @summary = opt[:message].to_s
     elsif @exception
       @summary = @exception.message
     end