Handle badly-formed Content-disposition headers

The www.nio.gov.uk web server insists on not quoting the `filename=`
portion of the content-disposition header, and `file` can't identify
proprietary file formats like XLS or PPT via the command line, so we
need to be more accepting of broken headers in order to get the correct
file name.

https://www.pivotaltracker.com/story/show/44945211