Fixes from comments associated with r12.

Author: jfisher

lib/gdata/http/response.rb

@@ -25,12 +25,13 @@ class Response
       # The headers of the HTTP response.
       attr_accessor :headers
 
+      # Converts the response body into a REXML::Document
       def to_xml
         if @body
           begin
             return REXML::Document.new(@body).root
           rescue
-            return nil
+            raise RuntimeError, "Response body not XML."
           end
         else
           return nil