Fix ternary in Nan::MaybeLocal<T>::FromMaybe<S>().

Make both arms return the same type to avoid build errors when S != T.

This works correctly in the V8 implementation of FromMaybe() but it
didn't until now in the nan fallback for v0.10 and v0.12.

nan_maybe_pre_43_inl.h

@@ -41,7 +41,7 @@ class MaybeLocal {
 
   template<typename S>
   inline v8::Local<S> FromMaybe(v8::Local<S> default_value) const {
-    return IsEmpty() ? default_value : val_;
+    return IsEmpty() ? default_value : v8::Local<S>(val_);
   }
 
  private: