Previously, we disallowed setting the empty string as a key in a map,
since at the time it seemed like doing so would allow all sorts of
unsavory bugs. In practice, I think this probably isn't actually true,
as I wasn't able to think of a scenario in which this bug would
materialize during the several moments I thought about it.

Plus, the code here to do sanity checking was wrong anyways.

Package param implements parameter parsing into a target struct (in much the
same way as encoding/json parses JSON into a struct). It targets the common
jQuery.param / Ruby on Rails style parameter serialization format.