tune logic and add comments

Author: charlesdong1991

pandas/io/formats/csvs.py

@@ -51,8 +51,11 @@ def __init__(self, obj, path_or_buf=None, sep=",", na_rep='',
         self.header = header
         self.index = index
         self.index_label = index_label
-        if not (self.header or self.index_label):
-            self.index_label = False
+        # set defualt index label to False if header is False and
+        # index_label is not None or False.
+        if not self.header:
+            if not self.index_label:
+                self.index_label = False
         self.mode = mode
         if encoding is None:
             encoding = 'ascii' if compat.PY2 else 'utf-8'