Some servers (e.g., wwwsearch.sourceforge.net) apparently send multiple
'Content-Length' headers, which causes setuptools to barf trying to convert
a '<length>, <length>' string to an integer.

This bug breaks installing 'mechanize', which lists wwwsearch.sourceforge.net
as its Download-URL, and therefore causes a bunch of Zope-related tests to fail
(e.g., https://mail.zope.org/pipermail/cmf-tests/2011-March/014576.html ).

The attached patch uses 'headers.getheaders('Content-Length')[0] to use only
the first value found.