httpc: temporary disable redirecting test case

Redirects can work improperly with libcurl-7.30 and older. CentOS 7
provides libcurl-7.29, so app-tap/http_client.test.lua fails on the
corresponding test case in CI. We need to disable the test case until
#4180 will be resolved.

While we are here fixed httpd.py to write a response at once, not
char-by-char (the typo from daf1ced8). See
gevent/pywsgy.py::process_result().

Related to #4119 and #4180.