Revert "hmset throws/errors out on non-string values. fixes #218"

Reverting because this was a documentation problem, not a problem with
the code. Performance-wise, this is faster than the approach in #345, though
it may cause users more trouble. This is okay, if someone opens an issue we
can point them to the docs.

This reverts commit b60e001fa08403240a6ecbf0c36d1c08ed8ccd9d.

Conflicts:

	index.js
	test.js