JCLOUDS-458:Multipart Related Upload

"If you have metadata that you want to send along with the data to upload, you can make a single multipart/related request. As with simple, media-only requests, this is a good choice if the data you are sending is small enough to upload again in its entirety if the connection fails."
Rewrite with assertj as follows:

```
assertThat(gcsObject.getAllMetadata())
   .contains(entry("customMetaKey", "customMetaValue"))
   .contains(entry("Adrian", "powderpuff"))
   .doesNotHave("adrian");
```