luci-base: do not coerce objects to strings

luci-base: do not coerce objects to strings

When getting values from the form, especially in a JSONMap,
some objects may be nested, and as such, it's not much use
to return '[object Object]'. Return the object instead, so
the caller can do something useful with it. This makes
multidimensional maps much easier to use.

Signed-off-by: Paul Donald <newtwen+github@gmail.com>