Types: Add method ObjectArray.as_generic

AFAIK, it is needed for reverse type lookups.

src/sqlalchemy_cratedb/type/array.py

@@ -95,6 +95,8 @@ def __init__(self, left, right, operator=operators.eq):
         self.operator = operator
 
 
+# TODO: Should this be inherited from PostgreSQL's
+#       `ARRAY`, in order to improve type checking?
 class _ObjectArray(sqltypes.UserDefinedType):
     cache_ok = True
 
@@ -140,5 +142,8 @@ def any(self, other, operator=operators.eq):
     def get_col_spec(self, **kws):
         return "ARRAY(OBJECT)"
 
+    def as_generic(self, **kwargs):
+        return sqltypes.ARRAY
+
 
 ObjectArray = MutableList.as_mutable(_ObjectArray)