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ARROW-1735: [C++] Test CastKernel writing into output array with non-zero offset

This uncovered some bugs. I inspected the other kernels that are untested and while they look fine, at some point we may want to add some more extensive unit tests about this

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

Closes #1369 from wesm/ARROW-1735 and squashes the following commits:

de41d929 [Wes McKinney] Test CastKernel writing into output array with non-zero offset