Fix nullable properties of TypeVariable types

We were forgetting to apply the property's nullability to the resolved type.

Fixes #521

kotlin-codegen/compiler/src/main/java/com/squareup/moshi/DelegateKey.kt

@@ -33,7 +33,7 @@ internal data class DelegateKey(
   private val type: TypeName,
   private val jsonQualifiers: Set<AnnotationMirror>
 ) {
-  val nullable get() = type.nullable || type is TypeVariableName
+  val nullable get() = type.nullable
 
   /** Returns an adapter to use when encoding and decoding this property. */
   fun generateProperty(

kotlin-codegen/compiler/src/main/java/com/squareup/moshi/metadata.kt

@@ -80,6 +80,7 @@ internal fun Type.asTypeName(
   val realType = when {
     hasTypeParameter() -> return getTypeParameter(typeParameter)
         .asTypeName(nameResolver, getTypeParameter, resolveAliases)
+        .let { if (nullable) it.asNullable() else it }
     hasTypeParameterName() -> typeParameterName
     hasAbbreviatedType() && !resolveAliases -> abbreviatedType.typeAliasName
     else -> className