BUG: Correct behavior for lists of tuples in unique, closes #4785

np.unique produces wrong results when passed a list of tuples and
no keyword arguments, as it fails to recognize it as a multidim
array, but handles it as a 1D array of objects. The only way around
this seems to be to completely eliminate the fast path for non-array
inputs using `set`.
Merge pull request #4788 from jaimefrio/unique_nonarray

BUG: Correct behavior for lists of tuples in unique, closes #4785