Ensure swift.source is set for DLO/SLO requests

SLO was setting it sometimes (the PUT path), but not others (the GET
path), and DLO just wasn't doing it at all. One could already
distinguish DLO/SLO internal requests by the user agent, but we should
set swift.source too.

Change-Id: I5d5b7dd49bd1522a9c830d0abd21fff92ae79a39
Remove duplicate doc entry for swob

Fixes swob module being referenced twice in misc.rst
resulting in duplicate sections in the doc.

Also fixes build_sphinx warning for section underline
too short in middleware.rst.

Change-Id: Ibe44895f933a6503ca04ccd3a084bc0cfd913213
Merge "Ensure swift.source is set for DLO/SLO requests"