v2.7.11-braze: Flush pending responses after exiting multi (#1)

* Flush pending responses after exiting multi

* Fix response flushing after noop

The `noop` operation was broken - it was assuming that all responses are
key/value pairs, as would be the case after a `get_multi`.

However, `noop` can be done at any time. In particular, it can be done
to conclude a `multi` block.

Correctly drain the responses by checking the key length and status, and
reading but discarding any response bodies.

This mirrors how the latest version of Dalli works:
https://github.com/petergoldstein/dalli/blob/main/lib/dalli/protocol/binary/response_processor.rb#L150

* Add test from https://github.com/petergoldstein/dalli/pull/844/files

* Only read body if there is something to read