incorrect use of mergeWith causing mutation to _fullLayout

[jonmmease]

While testing the barpolar trace in a FigureWidget I came across an error where certain Python update operations did not result in the expected change to the barpolar trace. After hours of Python/JavaScript debugging I found the source. The lodash mergeWith operation mutates the first argument and we were treating it as though it did not mutate any of the arguments. The fix is simply to pass an empty object as the first argument (7255511).

While debugging, I noticed that the createDeltaObject function was sometimes returning a delta object that included property values that were functions. This didn't seem to be causing any problems, but the result of createDeltaObject is the object that is serialized and sent back to the Python side so it's more consistent to remove these function properties here (1a04fbb).