(PUP-1389) Ensure wide strings have double null terminators

It seems that calling String#encode('UTF-16LE') sometimes adds only a single
NULL terminator in both ruby 1.9 and 2.0 on Windows. However, a wide string
is supposed to have two NULL terminators.

For reasons unknown, versions prior to 2012 seem to accept wide strings with
only a single NULL terminator, but in 2012 and 2012R2 the string is often times
contains a non-zero high byte, which causes Windows to read past the single
terminator, and attempt to access a path that does not exist.