Fix extra_info being reused across runs

58adec4677 was supposed to solve this, but ended up being an incomplete fix
Closes #727

yt_dlp/YoutubeDL.py

@@ -1166,7 +1166,7 @@ class YoutubeDL(object):
         for key, value in extra_info.items():
             info_dict.setdefault(key, value)
 
-    def extract_info(self, url, download=True, ie_key=None, extra_info={},
+    def extract_info(self, url, download=True, ie_key=None, extra_info=None,
                      process=True, force_generic_extractor=False):
         """
         Return a list with a dictionary for each video extracted.
@@ -1183,6 +1183,9 @@ class YoutubeDL(object):
         force_generic_extractor -- force using the generic extractor
         """
 
+        if extra_info is None:
+            extra_info = {}
+
         if not ie_key and force_generic_extractor:
             ie_key = 'Generic'