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ARROW-1328: [Python] Set correct Arrow type when coercing to milliseconds and passing explicit type

cc @fjetter

I'm planning to deprecate the `timestamps_to_ms` argument but it needs a patch in parquet-cpp first (so that nanoseconds/microseconds can be casted on the fly to milliseconds), so let's handle this fix first

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

Closes #941 from wesm/ARROW-1328 and squashes the following commits:

4632dc36 [Wes McKinney] Set correct Arrow type when coercing to milliseconds and passing explicit type (which may not be milliseconds)