dashdemux: add support for HTTP HEAD method of time sync

dashdemux: add support for HTTP HEAD method of time sync

The urn:mpeg:dash:utc:http-head:2014 method of time synchronisation
uses an HTTP HEAD request to a specified URL and then parses the
Date: HTTP response header.

This commit adds support to dashdemux for this method of time
synchronisation by making a HEAD request and then parsing the Date:
response.

This commit adds support to gstfragment to return the HTTP headers
and to uridownloader to support HEAD requests. To avoid creating a
new API, the RANGE get function is re-used (abused?) with start=-1
and end=-1 to indicate a HEAD request.

https://bugzilla.gnome.org/show_bug.cgi?id=752413