"string" isn't a valid column type for ActiveRecord

Conflicts:
	activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
Updated comment to mention the enum mapping class method [ci skip]
directly create the ARel AST

rather than allocating a new Relation, just make the AST directly
rm dup code

https://github.com/rails/rails/blob/e64a83cdbd3ce836f3339c600c1006b20c23d8fd/activerecord/test/cases/relation/merging_test.rb#L80-91