Check if iterator has length

If an iterable is passed into `FuncAnimation` check if it has
`__len__` before trying to take it's length

closes #1769
added test for code path where the object passed into `frames` is
does not have a length.
corrected test function name
added test_animation back in to default_test_modules so that the
iterator test actually runs
Merge pull request #2634 from tacaswell/animation_iter_length

address FuncAnimantion trying to take lengths of generators