sniffing: Document the specification we are targeting
sniffing: Implement the check-apache-bug flag

Run the text or binary algorithm when some specific text/plain Content-Types
are provided, since older versions of apache would send that type for binary
files. http://mimesniff.spec.whatwg.org/#dfnReturnLink-0
sniffing: Implement handling of the X-Content-Type-Options header
sniffing: Adjust the general unknown MIME type algorithm

This change adjusts the pattern matching table to the current form of
the MIMESNIFF spec, adding a check for a tag-terminating byte and
using the formally defined sniff-scriptable flag.
sniffing: test that it allows leading whitespace in doctype headers

https://bugzilla.gnome.org/show_bug.cgi?id=715126
sniffing: Add comment to the XML types bit
sniffing: Bring feed vs HTML up-to-date with the MIMESNIFF spec

* decide on that before doing the image sniffing to match the spec
* use const char* and g_str_has_prefix for comparisons to make it
  more legible
* deal with rdf:RDF tags
sniffing: Bring image sniffing up-to-date with the MIMESNIFF spec
sniffing: Add audio/video sniffing
Merge branch 'content-sniffing-update'

Update SoupContentSniffer (and sniffing-test) to match the current
version of the MIME sniffing spec.

https://bugzilla.gnome.org/show_bug.cgi?id=648849
https://bugzilla.gnome.org/show_bug.cgi?id=715126