Handle sequel qualified identifier join

[alex-lairan]

PostgreSQL has the possibility to handle multiple schemas.

Within ROM-SQL it is possible to query the table using Sequel::SQL::QualifiedIdentifier object instead of a symbol.

The join doesn't recognize Sequel::SQL::QualifiedIdentifier so this PR is just about fixing this to allow the join query to be made.

TODO :

• Tests
• Verify how SQL schema and other schema works together with a Sequel object instead of a symbol

Ideas of improvement :

• Create an object that encapsulates the Sequel part so that it doesn't leak into the ROM Relation code. e.g : ROM::SQL::Postgres::SchemaScoppedTableName