Use setdefault in handleHeader

New submission from johnnyg <None>:

In twisted/web/client.py, the function handleHeader (on L66) does:

{{{
#!python
l = self.headers[key] = self.headers.get(key, [])
}}}

which is less clear and less efficient than doing:

{{{
#!python
l = self.headers.setdefault(key, [])
}}}

I've also attached a patch if you're feeling lazy.

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Type     : defect
Component: web
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/4216