[fix] duckduckgo lite engine: set HTTP header 'Referer'

We have had problems with this before, the bot protection from ddg-lite seems to
have included this referer in the rating [1][2].

From reverse engineering:

- The Referer ``https://google.com/`` was set in commt 257dc7d --> DDG lite
  does not like this referer anymore!

- The 'Referer' header is only set on second and follow up pages but not on the
  first page

- The vqd value is not needed on the first page, the ddg-lite client sets this
  value only on follow up pages / this can help to reduce the vqd requests from
  SearXNG.

Related to 'Referer' header & ddg requests:

[1] #2161
[2] #2081

Closes: #2796
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/engines/duckduckgo.py

@@ -222,7 +222,6 @@ def request(query, params):
     # link again and again ..
 
     params['headers']['Content-Type'] = 'application/x-www-form-urlencoded'
-    params['headers']['Referer'] = 'https://google.com/'
 
     # initial page does not have an offset
     if params['pageno'] == 2:
@@ -248,6 +247,10 @@ def request(query, params):
         params['data']['nextParams'] = form_data.get('nextParams', '')
         params['data']['v'] = form_data.get('v', 'l')
 
+        # request needs a vqd argument
+        params['data']['vqd'] = get_vqd(query)
+        params['headers']['Referer'] = 'https://lite.duckduckgo.com/'
+
     params['data']['kl'] = eng_region
     params['cookies']['kl'] = eng_region