[Twisted-web] backport if-modified-since bugfix from web2 to web

[Stefano Debenedetti]

Hello, 

As noted in a web2 docstring:

    # Ancient versions of netscape and *current* versions of MSIE send
    #   If-Modified-Since: Thu, 05 Aug2004 12:57:27 GMT; length=123
    # which is blantantly RFC-violating and not documented anywhere

While we wait for web2, the attached patch [1] against SVN trunk revision13528
makes twisted.web fix the issue the same way as twisted.web2.

ciao
ste

[1] http://twistedmatrix.com/bugs/issue976