get_url pass incorrect If-Modified-Since header(#67417) (#67419)

Fix #67417. HTTP header value of `If-Modified-Since` set by `get_url` does not follow HTTP protocol.

(cherry picked from commit 1097694355)

changelogs/fragments/67417-get_url-incorrect-if-modified-since.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - get_url pass incorrect If-Modified-Since header (https://github.com/ansible/ansible/issues/67417)

lib/ansible/module_utils/urls.py

@@ -1274,7 +1274,7 @@ class Request:
             request.add_header('cache-control', 'no-cache')
         # or we do it if the original is more recent than our copy
         elif last_mod_time:
-            tstamp = rfc2822_date_string(last_mod_time.timetuple())
+            tstamp = rfc2822_date_string(last_mod_time.timetuple(), 'GMT')
             request.add_header('If-Modified-Since', tstamp)
 
         # user defined headers now, which may override things we've set above

test/units/module_utils/urls/test_Request.py

@@ -415,7 +415,7 @@ def test_Request_open_last_mod(urlopen_mock, install_opener_mock):
     args = urlopen_mock.call_args[0]
     req = args[0]
 
-    assert req.headers.get('If-modified-since') == now.strftime('%a, %d %b %Y %H:%M:%S -0000')
+    assert req.headers.get('If-modified-since') == now.strftime('%a, %d %b %Y %H:%M:%S GMT')
 
 
 def test_Request_open_headers_not_dict(urlopen_mock, install_opener_mock):