Fixed two compilation bugs involving toLower()

Since the import `std.string` aliases `toLower` to `std.uni.toLower` (http://dlang.org/phobos/std_string.html#.toLower), and the import `std.ascii` also implements `toLower(Char)`, DMD was confused about which one to use, and threw a compilation error for metadata.d & query.d.

This PR disambiguates its use by explicitly specifying `std.ascii.toLower` for the two calls where a `Char` is passed in.