Change the ie_name of YoutubeSearchDateIE

It produced a duplicate entry when listing the extractors with '--list-extractors' and generates noise in the commit log when generating the supported sites webpage (like in 09f355f73bf1657ecacfd05eda21d2c4bf1cc4a8)

youtube_dl/extractor/youtube.py

@@ -1765,6 +1765,7 @@ class YoutubeSearchIE(SearchInfoExtractor):
         return self.playlist_result(videos, query)
 
 class YoutubeSearchDateIE(YoutubeSearchIE):
+    IE_NAME = YoutubeSearchIE.IE_NAME + ':date'
     _API_URL = 'https://gdata.youtube.com/feeds/api/videos?q=%s&start-index=%i&max-results=50&v=2&alt=jsonc&orderby=published'
     _SEARCH_KEY = 'ytsearchdate'
     IE_DESC = u'YouTube.com searches, newest videos first'