Propagete context of returned deferred object in Deferred.then(). Fixes #13160
Eliminate redundancy of Deferred.then().
Tweaks the logic in the 304-related unit tests so that they pass for versions of Opera that support manual 304 handling as well as for those that don't
Adding some more test coverage for .finish()
strengthens and dries 304 unit tests
Opera 12.0 still implements an older version of the DOM so cloneNode cannot be called without an argument
Adjusts unit test regarding exception in injected scripts so that it accounts for the exception being thrown in an event loop for remote scripts
Propagate context of returned deferred object in Deferred.then(). Fixes #13160.