These test were failing because they made live requests to httpbin.org,
which is serving intermittent 500s.
In general we strive to make our test and doctests interact with a local
test server rather than a live website. For doctests we do this with
some sleight of hand in a hidden `ureq::is_test(true)`, which swaps out
the default agent returned by ureq::agent. However, since the middleware
doctests rely on constructing a custom agent with the middleware, they
can't rely on this.
I suspect we can get around this problem but it may take some thinking.
Similarly disable the AgentBuilder::user_agent and redirects doctests
and the agent_set_header unittest.
905bf8f0b2