images.

Add response parameters to custom_metadata streaming request for Node and PHP
clients.

The Java server does not respond with separate initial and trailing
metadata when there is no response data - it is only emiting the
requested trailing metadata. Adding the response parameters to the test
(in accordance with the specification) avoids this, but I will open a
separate issue to investigate the Java behavior.

This should allow our tests to build on newer virtualenvs.

Fixes #8285, allows us to work on later versions of virtualenv without
getting problems with too-recent python packaging versions.