to_stata uint16

Simple changes to io/stata.py to write unsigned integers to Stata files. Sorry about the poor commit messages--I'm still new to git and wasn't able to correct it cleanly.

(See issue #7365)
This is not safe and can lead to data loss.  Only supported Stata data types should be used here, and Stata does not support unsigned types.  The correct method is to first perform all casting and then use only the case, Stata-safe data types when writing the data type.