Environment details

1. FakeStorageRpc
2. OS type and version: Win 10
3. Java version: java 8
4. google-cloud-nio version(s): 0.121.2

Steps to reproduce

1. Add object with name "test-object" to bucket "first-bucket",
2. Add object with name "test-object" to bucket "second-bucket".
3. List objects in "first-bucket" with "test-" prefix.
4. Both "test-object" files from "first-bucket" and "second-bucket" are returned.

Any additional information below

The list method in the FakeStorageRpc does not take into account the bucket metadata:

    for (StorageObject so : metadata.values()) {
      if (!so.getName().startsWith(prefix)) {                       // <---------- here's the problem
        continue;
      }
      if (processedAsFolder(so, delimiter, prefix, folders)) {
        continue;
      }
      so.setSize(size(so));
      values.add(so);
    }

Technically, just adding an extra check to compare the so.getBucket() with bucket should be enough to resolve the issue.