Raise NoMethodError and convert the field name to a String.
Added specs for Field.
Deleted the old Fragment specs.
Updated the SQL helper method specs.
Added new specs for SQL::Function.
Always convert the Field name to a String.
Call super from method_missing.
Added specs for SQL::Fields.
Include Statements into Clause for the UNION clause.
Added specs for Clauses.
Added specs for Clause.