Work around iOS7 memory trouble by not applying the contextId performance optimization when context==this. The optimization does nothing in this case anyway. Fixes #2122
We set the context value to context || this in addEventListener, so we don't need to do so here.
Fix a potential event bug due to bad cleanup.
Improve this test and fix removeEventListener.
Merge pull request #2149 from danzel/ios7-2122

Work around iOS7 memory trouble