Do not load association during initialization of cursor.

[simi]

Hello, I have spotted when cursor is used on association the association is actually loaded (sql query is triggered).

I have found out it is related to type check in iterate_type. Using == to compare type triggers the association query. I have added type check of ŧype.class to avoid calling equality operator on active record relation.

There's need to access @type via instance_variable_get on cursor in specs, but I think it is not worth it to expose type to public just because of specs.

before (cursor is initialized and prices relation is loaded)

[4] pry(#<TestPostgresqlCursor>)> cursor = Product.first.prices.each_instance;nil
D, [2020-01-17T00:14:47.381230 #1955109] DEBUG -- :   Product Load (0.7ms)  SELECT "products".* FROM "products" ORDER BY "products"."id" ASC LIMIT $1  [["LIMIT", 1]]
D, [2020-01-17T00:14:47.382515 #1955109] DEBUG -- :   Price Load (0.3ms)  SELECT "prices".* FROM "prices" WHERE "prices"."product_id" = $1  [["product_id", 1]]
=> nil

after (cursor is initialized and prices relation is not loaded)

[2] pry(#<TestPostgresqlCursor>)> cursor = Product.first.prices.each_instance;nil                                                                                                             
D, [2020-01-17T00:13:48.292153 #1954698] DEBUG -- :   Product Load (0.3ms)  SELECT "products".* FROM "products" ORDER BY "products"."id" ASC LIMIT $1  [["LIMIT", 1]]
=> nil