extract_regex: encoding arg defaults to 'utf-8'

Sometimes it is not neccessary to pass the encoding argument. For
example, when the text argument is unicode. So set a default encoding.

Signed-off-by: Ping Yin <pkufranky@gmail.com>

scrapy/utils/misc.py

@@ -42,7 +42,7 @@ def load_object(path):
 
     return obj
 
-def extract_regex(regex, text, encoding):
+def extract_regex(regex, text, encoding='utf-8'):
     """Extract a list of unicode strings from the given text/encoding using the following policies:
 
     * if the regex contains a named group called "extract" that will be returned