TST/REF: collect tests from test_api into method-specific files
I was surprised we didnt already have a test_copy

Also an easter egg:

```
    def test_keys(self, datetime_series):	    
        # HACK: By doing this in two stages, we avoid 2to3 wrapping the call
        # to .keys() in a list()	
        getkeys = datetime_series.keys	
        assert getkeys() is datetime_series.index
```