Use '%20' instead of '+' to encode a space.

While [application/x-www-form-urlencoded](http://www.w3.org/TR/html5/forms.html#application/x-www-form-urlencoded-encoding-algorithm) encodes a space as `'+'`, `'%20'` is the more common and more universally accepted encoding. For examaple, JavaScript's own `encodeURIComponent` uses `'%20'`. Similarly, `decodeURIComponent` will properly decode `'%20'` into a space but does not expand `'+'`.