Fix SQLCastExpression for MySQL (#4)

Currently `SQLCastExpression` doesn't work on MySQL because of syntax
differences. This fixes it.

Author: ptoffy

Sources/SQLKitExtras/SQLKitExtras/Expressions/SQLCastExpression.swift

@@ -29,7 +29,17 @@ public struct SQLCastExpression: SQLExpression {
 
     /// See `SQLExpression.serialize(to:)`.
     public func serialize(to serializer: inout SQLSerializer) {
-        SQLFunction("CAST", args: SQLAlias(self.original, as: self.desiredType))
+        let desiredType: any SQLExpression = if 
+            serializer.dialect.name == "mysql",
+            let ident = self.desiredType as? SQLIdentifier,
+            ident.string.allSatisfy({ $0.isASCII && ($0.isLowercase || $0.isUppercase || $0.isWholeNumber || $0 == "_") })
+        {
+            SQLRaw(ident.string)
+        } else {
+            self.desiredType
+        }
+
+        SQLFunction("CAST", args: SQLAlias(self.original, as: desiredType))
             .serialize(to: &serializer)
     }
 }

Tests/SQLKitExtrasTests/FluentSQLKitExtrasTests.swift

@@ -302,6 +302,9 @@ struct FluentSQLKitExtrasTests {
         func castExpression() {
             #expect(serialize(.cast(\FooModel.$field, to: "text")) == #"CAST("foos"."field" AS "text")"#)
             #expect(serialize(.cast(\FooModel.$field, to: .unsafeRaw("text"))) == #"CAST("foos"."field" AS text)"#)
+            
+            #expect(MockSQLDatabase(dialect: "mysql").serialize(.cast(\FooModel.$field, to: "text")).sql == #"CAST("foos"."field" AS text)"#)
+            #expect(MockSQLDatabase(dialect: "postgresql").serialize(.cast(\FooModel.$field, to: "text")).sql == #"CAST("foos"."field" AS "text")"#)
         }
     }
 }

Tests/SQLKitExtrasTests/SQLKitExtrasTests.swift

@@ -259,6 +259,9 @@ struct SQLKitExtrasTests {
             #expect(serialize(.cast("foo", to: "text")) == #"CAST("foo" AS "text")"#)
             #expect(serialize(.cast(.column("foo"), to: "text")) == #"CAST("foo" AS "text")"#)
             #expect(serialize(.cast(.column("foo"), to: .unsafeRaw("text"))) == #"CAST("foo" AS text)"#)
+
+            #expect(MockSQLDatabase(dialect: "mysql").serialize(SQLCastExpression(.column("foo"), to: "text")).sql == #"CAST("foo" AS text)"#)
+            #expect(MockSQLDatabase(dialect: "postgresql").serialize(SQLCastExpression(.column("foo"), to: "text")).sql == #"CAST("foo" AS "text")"#)
         }
 
         @Test