Improve support for non-integer primary keys

Only skip the regular AR default find if the primary key is an integer
AND the given ID is also an integer (need to support UUID primary keys)

Resolves #461
Resolves #455

Signed-off-by: Norman Clarke <norman@njclarke.com>
Memoize primary key type.

Adds test to ensure a string primary key is handled properly

Signed-off-by: Norman Clarke <norman@njclarke.com>
Updated changelog