Add complex numbers to the supported data types for UnsortedSegmentProd

In the kernel implementation both UnsortedSegmentProd and UnsortedSegmentSum
supports complex numbers. However, unlike UnsortedSegmentSum, the op
of UnsortedSegmentProd does not register complex number types in math_ops.cc.

This fix adds the supported complex number types to math_ops.cc,
and enables test cases for it.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>