Switch http client_test to use google.com/robots.txt to avoid redirect loop.

www.google.com seems to be redirecting requests in an infinite loop. I haven't tracked down whether it's their code or this code that is causing it. This is just a quick fix so that this test passes.

APPROVED=r
DELTA=4  (0 added, 0 deleted, 4 changed)
OCL=30178
CL=30210