fixed the null model within thumbnail request

The thumbnail request model is null when use "GenericRequestBuilder.thumbnail(GenericRequestBuilder<?, ?, ?, TranscodeType>)", it cause the thumbnail request do nothing.

For example
```java
DrawableRequestBuilder<Uri> fullRequest = Glide.with( context ).fromUri();
DrawableRequestBuilder<Uri> thumbnailRequest = Glide.with( context ).fromUri().override( 160, 160 );
fullRequest.thumbnail( thumbnailRequest );
fullRequest.load( uri ).into( target );
```
Then, in the "com.bumptech.glide.GenericRequestBuilder.buildRequestRecursive(Target<TranscodeType>, ThumbnailRequestCoordinator)"
```java
Request thumbRequest = thumbnailRequestBuilder.buildRequestRecursive(target, coordinator); // created request's model is null
```