Can we check to see if ProxyFromEnvironment doesnt return err?
Never mind. Got mixed up with the function https://golang.org/src/net/http/transport.go#L266

LGTM for sure

I think this is the way it's supposed to be used (also see https://golang.org/pkg/net/http/#ProxyFromEnvironment, and the example on that page)

A nil URL and nil error are returned if no proxy is defined in the environment, or a proxy should not be used for the given request, as defined by NO_PROXY.

As a special case, if req.URL.Host is "localhost" (with or without a port number), then a nil URL and nil error will be returned.

So an error is only returned in situations where no proxy should be used (in which case nil is returned)

PowerPC node has some issues; https://jenkins.dockerproject.org/job/Docker-PRs-powerpc/8278/console

17:36:15 Step 6/31 : RUN curl -fsSL "https://golang.org/dl/go${GO_VERSION}.linux-ppc64le.tar.gz" 	| tar -xzC /usr/local
17:36:15  ---> Running in 4abd3531a716
17:36:16 curl: (56) SSL read: error:00000000:lib(0):func(0):reason(0), errno 104
17:36:16 
17:36:16 gzip: stdin: unexpected end of file
17:36:16 tar: Unexpected EOF in archive
17:36:16 tar: Unexpected EOF in archive
17:36:16 tar: Error is not recoverable: exiting now

Added a unit test.

I tried adding a more complete test using the NewHTTPEventCollectorMock mock in this package but that won't work because golang refuses to use a proxy for any localhost/127.x.x.x url (https://github.com/golang/go/blob/release-branch.go1.9/src/net/http/transport.go#L1218-L1225)

Since the rest of this code is part of the stdlib, all we really need to test is that the transport has the correct proxy factory, which is covered by this test.