Fix a bug that select --query _key:XXX for Int*/UInt* doesn't work

It is caused only TABLE_PAT_KEY because GRN_OP_GET_VALUE in
grn_expr_exec() uses grn_obj_get_value_() (see the last "_") instead
of grn_obj_get_value(). get_value_() is lighter version of
get_value(). get_value_() doesn't support key decoding. TABLE_PAT_KEY
encodes int/uint/geo point/float key. So we can't use get_value_() for
Int*/UInt*/*GeoPoint/Float.

This change uses get_value() to get column value. (_key is a pseudo
column.)