!= was being used instead of == in conditional

With this operator, this conditional was checking for whether the
global proxy ID existed in a std::set that used to ensure uniqueness
of the proxy in the collection. Instead, it should check whether that
ID wasn't in the std::set.

Added test for this fix

Change-Id: I0a2b5bb614acff6bdc557ba9da7c1ef85c501691
Merge topic 'fix_get_proxies' into next

645ac1ea != was being used instead of == in conditional