Fix double-encoding of spaces in query parameter keys in ->uri_for

Fix double-encoding of spaces in query parameter keys in ->uri_for

Commit 5c779e9841d052721162a48ad96fdbda2acd1035 moved half of the key-
encoding part into the map over the values, but left the `s/ /+/g` in
place, causing spaces in query parameter keys to first be converted to
+, then to %2B.

This moves the current key encoding block out of the inner map and
removes the premature `s/ /+g/`.