Fix mocking of HTTPClient.request() method.

The test_extend_list_exceed_max_uri_len test case mocks
the request() method of HTTPClient class so that it raises
the RequestURITooLong exception. But the real method can't
possibly raise it (Client.do_request() does this instead).
Mocks should emulate the behavior of methods they stub out
and not change it in any way.

Fixes bug 1192197.

Change-Id: I62b2db111ef251f95eb9aa9c9cc00b53fdbccc68
Merge "Fix mocking of HTTPClient.request() method."