Store setUserValue as string in cache

We cache the values we set in the setUserValue function.
However since the values are strings in the database we check if a value
is equal with string comparison

Now if the function was called with a $value of int or float. It would
be stored in the DB (and thus converted to string) and in the cache (not
converted thus as int/float).

Now if another call comes in that sets it to the same value (I'm looking
at you LDAP!). The check would fail since we would be comparing
int/float to string which fails by definition.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>