Type inference with list of lists in DataFrame constructor

[wesm]

per @aristotle137

In [1216]: l = [[1, 'a'], [2, 'b']]
     ...: df = DataFrame(data = l, columns = ["num", "str"])
     ...:

In [1217]: df.dtypes
Out[1217]: num    object
               str    object

[wesm]

sorted this out in the above commit. thanks for pointing it out!