[mod] image proxy: allow binary/octet-stream mime type

The Petal Search Images engine sends the thumbnails with the binary/octet-stream mime type.

searx/webapp.py

@@ -1163,7 +1163,9 @@ def image_proxy():
                 return '', resp.status_code
             return '', 400
 
-        if not resp.headers.get('Content-Type', '').startswith('image/'):
+        if not resp.headers.get('Content-Type', '').startswith('image/') and not resp.headers.get(
+            'Content-Type', ''
+        ).startswith('binary/octet-stream'):
             logger.debug('image-proxy: wrong content-type: %s', resp.headers.get('Content-Type', ''))
             return '', 400