bugfix(!biv) : bing-video do not like "older" User-Agents

When selecting other languages than 'en', bing-video did not handle the language
correct and gave very bad results.  Since User-Agent is normaly rotated in
searx, the behavior of a !biv search was unpredictable and paging was broken.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>