-J -O: use -O name if no Content-Disposition header comes!

A regression between 7.22.0 and 7.23.0 -- downloading a file with the
flags -O and -J results in the content being written to stdout if and
only if there was no Content-Disposition header in the http response. If
there is a C-D header with a filename attribute, the output is correctly
written.

Reported by: Dave Reisner
Bug: http://curl.haxx.se/mail/archive-2011-11/0030.html