Fix http_content_length_test by not closing the server from the client.

Fix http_content_length_test by not closing the server from the client.

This would invoke socket.destory calls, leading to the socket dying
normally, before an error can occour.

BUG=https://code.google.com/p/dart/issues/detail?id=10934
R=sgjesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=23802

[Søren Gjesse, 2013-06-10 08:06:02]

lgtm

[Anders Johnsen, 2013-06-10 10:20:08]

Committed patchset #1 manually as r23802 (presubmit successful).