Fix #1190 by marking RequestOptions transient

Also try to prevent similar problems in the future by checking that
we only use the reflection-based type adapter for classes in `com.stripe.`.

src/main/java/com/stripe/model/StripeCollection.java

@@ -50,7 +50,7 @@ public abstract class StripeCollection<T extends HasId> extends StripeObject
 
   @Getter(onMethod_ = {@Override})
   @Setter(onMethod = @__({@Override}))
-  private RequestOptions requestOptions;
+  private transient RequestOptions requestOptions;
 
   @Getter(onMethod_ = {@Override})
   @Setter(onMethod = @__({@Override}))

src/main/java/com/stripe/net/ApiResourceTypeAdapterFactoryProvider.java

@@ -18,6 +18,7 @@ final class ApiResourceTypeAdapterFactoryProvider {
     factories.add(new BalanceTransactionSourceTypeAdapterFactory());
     factories.add(new ExternalAccountTypeAdapterFactory());
     factories.add(new PaymentSourceTypeAdapterFactory());
+    factories.add(new ReflectionCheckingTypeAdapterFactory());
   }
 
   public static List<TypeAdapterFactory> getAll() {

src/main/java/com/stripe/net/ReflectionCheckingTypeAdapterFactory.java

@@ -0,0 +1,26 @@
+package com.stripe.net;
+
+import com.google.gson.Gson;
+import com.google.gson.TypeAdapter;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.internal.bind.ReflectiveTypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+
+/**
+ * {@link TypeAdapterFactory} that checks that we don't use {@link ReflectiveTypeAdapterFactory} accidentally
+ * for classes outside {@code com.stripe} packages. This usually happens when we forget to mark a field
+ * {@code transient}.
+ */
+class ReflectionCheckingTypeAdapterFactory implements TypeAdapterFactory {
+  @Override
+  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    if (!type.getType().getTypeName().startsWith("com.stripe.")) {
+      TypeAdapter<T> adapter = gson.getDelegateAdapter(this, type);
+      if (adapter instanceof ReflectiveTypeAdapterFactory.Adapter) {
+        throw new IllegalArgumentException(
+            "Refusing to create type reflection-based type adapter for external class: " + type);
+      }
+    }
+    return null;
+  }
+}