intl 591 fix unit source field corruption

The proposed patch consists in adding some logic to the servlet that handles submission of translations so it would skip|ignore updates submission request for source field attributes (coming from the webapp). It looks pretty straight forward. 

Testing Done:
- I was able to interactive test the fix in my playground and is looking good, no side effects. 
- I tried to write some unit test for this change but I wasn't able to get the test suite to run, following the instructions in http://docs.translatehouse.org/projects/pootle/en/stable-2.5.0/developers/testing.html
It seems to failed when trying to create the sandbox DB for testing https://gist.github.com/jagg81/31468fe3bd0d69262ff5