Plaintext files are forced to be downloaded as attachments (BB-706)

Given a file with a text MIME type, instead of displaying it in the browser, Safari is forced to download it as an attachment:

{{{
$ curl -I http://bitbucket.org/parren/hg-pbranch/raw/pbranch-graphdesc/hgext/pbranch.py\\
HTTP/1.1 200 OK
Server: nginx/0.6.32
Date: Mon, 18 May 2009 12:30:46 GMT
Content-Type: text/x-python
Transfer-Encoding: chunked
Connection: keep-alive
Vary: Cookie
Content-Disposition: attachment
}}}

The Content-Disposition header should be set to inline or dropped completely for any text/* MIME type that isn't executed by the browser.