Don't use SObject.custom_table? to infer BelongsToAssociation#sfdc_assoc...

...iation_field. It's already handled in ActiveForce::Table

This fixes a bug where using `includes` on relationships of namespaced SObjects would generate invalid SOQL. The namespace was being included in the table name in the query erroneously. ex `SELECT Id, Salesforce::Account.Id FROM Opportunity ...`