Added a static method to turn a Map<?, ?> straight into a query string.

Sometimes we have a Map already built via other methods, and there's
really no reason to copy each key/value into a Multimap prior
to encoding them.

Accept a Map<?, ?> and just take the small extra step of toStringing
every key and value, otherwise we'd have to transform the map
values and that's a bunch of extra wrapping.
I didn't end up using these imports.