ARROW-1345: [Python] Test conversion from nested NumPy arrays with smaller int, float types
ARROW-1345: [Python] Test conversion from nested NumPy arrays with smaller int, float types

This also resolves ARROW-2008

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #1643 from wesm/ARROW-1345 and squashes the following commits:

a0997104 [Wes McKinney] Add another test
8af33486 [Wes McKinney] Add unit test for ARROW-1345

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