Bundling sub-response handling is broken for actual proto objects

[bjwatson]

This line of code is invalid for proto repeated fields, which is what we're attempting to set as the sub-response field for bundling responses. This is the root cause of googleapis/google-cloud-python#1869.

We did not catch this earlier, because we are testing with named tuples. As part of this fix, we might want to switch our tests to use a simple proto object.