Fix problems with += stmts.
Fix problems with += stmts.

flurischt/jedi,tjwei/jedi,WoLpH/jedi,dwillmer/jedi,mfussenegger/jedi,jonashaag/jedi,tjwei/jedi,jonashaag/jedi,flurischt/jedi,mfussenegger/jedi,dwillmer/jedi,WoLpH/jedi