Support header-related params in test cases

Header-related params are parsed into `std_headers` rather than
the `params` dict, but this wasn't respected for test cases.

test/test_download.py

@@ -34,6 +34,7 @@
     DownloadError,
     ExtractorError,
     format_bytes,
+    std_headers,
     UnavailableVideoError,
 )
 from youtube_dl.extractor import get_info_extractor
@@ -125,6 +126,17 @@ def print_skipping(reason):
             params.setdefault('extract_flat', 'in_playlist')
             params.setdefault('skip_download', True)
 
+        if 'user_agent' in params:
+            std_headers['User-Agent'] = params['user_agent']
+
+        if 'referer' in params:
+            std_headers['Referer'] = params['referer']
+
+        for h in params.get('headers', []):
+            h = h.split(':', 1)
+            if len(h) > 1:
+                std_headers[h[0]] = h[1]
+
         ydl = YoutubeDL(params, auto_init=False)
         ydl.add_default_info_extractors()
         finished_hook_called = set()