Replace a default string value with None.

The previous version was redundant.

odo/backends/csv.py

@@ -103,7 +103,7 @@ def open_file(path, *args, **kwargs):
         f.close()
 
 
-def infer_header(path, nbytes=10000, encoding='utf-8', **kwargs):
+def infer_header(path, nbytes=10000, encoding=None, **kwargs):
     if encoding is None:
         encoding = 'utf-8'
     with open_file(path, 'rb') as f: