Update whitelisted_generic_onebox.rb (#400)

Update whitelisted_generic_onebox.rb (#400)

add poster to videos from go:image

diff --git a/lib/onebox/engine/whitelisted_generic_onebox.rb b/lib/onebox/engine/whitelisted_generic_onebox.rb
index bb378c6..d2da5dd 100644
--- a/lib/onebox/engine/whitelisted_generic_onebox.rb
+++ b/lib/onebox/engine/whitelisted_generic_onebox.rb
@@ -343,7 +343,7 @@ module Onebox
                    width='#{data[:video_width]}'
                    height='#{data[:video_height]}'
                    style='max-width:100%'
-                   controls=''>
+                   controls='' poster='#{data[:image]}'>
               <source src='#{escaped_src}'>
             </video>
           HTML

GitHub sha: 0f827330