get-with-headers: don't block indefinitely if the server had an internal error

If the server had an internal error and returned 500, there's nothing
to read, so "response.read()" blocks indefinitely.  Only output the
response if there's really a response.

tests/get-with-headers.py

@@ -43,6 +43,7 b' def request(host, path, show):'
             print "%s: %s" % (h, response.getheader(h))
     if not headeronly:
         print
+        if response.status != 500:
         data = response.read()
         sys.stdout.write(data)