Send the correct Content-Type header for conditional requests

New submission from garetjax <None>:

The `twisted.static.File.render` method correctly returns a `NOT_MODIFIED` response for conditional `If-Modified-Since` requests but does not set the correct `Content-Type` header (leaving the default `text/html`).

As the [http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.5.3 section 13.5.3 of the HTML 1.1 spec] says, any headers sent as a response to a conditional request should replace the cached ones, this means that a response to a conditional request will always change the `Content-Type` of the cached resource to `text/html`.

The incriminated code chunk is here:[[BR]]
http://twistedmatrix.com/trac/browser/tags/releases/twisted-9.0.0/twisted/web/static.py#L620



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Type     : defect
Component: web
Keywords : web request conditional content-type
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/4156