Fix the transformer being cleared for Codable properties

This should fix issue #27.

The `transformerName` got reset as part of processing
the model options.

Sources/ManagedModels/SchemaCompatibility/NSAttributeDescription+Data.swift

@@ -165,7 +165,11 @@ public extension NSAttributeDescription {
     if let hashModifier { versionHashModifier = hashModifier }
     if let defaultValue { self.defaultValue   = defaultValue }
     isOptional = valueType is any AnyOptional.Type
+
+    assert(valueTransformerName == nil)
+    valueTransformerName = nil
     if valueType != Any.self { self.valueType = valueType }
+
     setOptions(options)
   }
 }
@@ -177,7 +181,6 @@ private extension NSAttributeDescription {
     allowsExternalBinaryDataStorage   = false
     isIndexedBySpotlight              = false
     isTransient                       = false
-    valueTransformerName              = nil
     if #available(iOS 15.0, macOS 12.0, tvOS 15.0, watchOS 8.0, *) {
       allowsCloudEncryption = false
     }
@@ -193,8 +196,10 @@ private extension NSAttributeDescription {
         case .ephemeral: isTransient = true
 
         case .transformableByName(let name):
+          assert(valueTransformerName == nil)
           valueTransformerName = name
         case .transformableByType(let type):
+          assert(valueTransformerName == nil)
           valueTransformerName = NSStringFromClass(type)
 
         case .allowsCloudEncryption: // FIXME: restrict availability

Tests/ManagedModelTests/CodablePropertiesTest.swift

@@ -28,6 +28,7 @@ final class CodablePropertiesTests: XCTestCase {
       defaultValue: nil
     )
     XCTAssertEqual(attribute.name, "sip")
+    XCTAssertEqual(attribute.attributeType, .transformableAttributeType)
 
     let transformerName = try XCTUnwrap(
       ValueTransformer.valueTransformerNames().first(where: {