change corpusStats to return a pandas.DataFrame by default

For corpusQuery this makes no sense.

Resolves #12

Change-Id: I7e58f47f969131ec0ef8dc4cbb6a041d665207b9
diff --git a/KorAPClient/__init__.py b/KorAPClient/__init__.py
index 19ae5c7..40db1a0 100644
--- a/KorAPClient/__init__.py
+++ b/KorAPClient/__init__.py
@@ -43,9 +43,9 @@
     def corpusStats(self, *args, **kwargs):
         """Query the size of the whole corpus or a virtual corpus specified by the vc argument.
 
-            - vc = ""
-            - verbose = kco@verbose
-            - as.df = False
+        - **vc** (default = "")
+        - **verbose** (default = kco@verbose)
+        - **as.df** (default = True)
 
         Returns:
             `DataFrame`|`RS4`
@@ -57,8 +57,10 @@
             12150897
             ```
         """
+        default_kwargs = {"as.df": True}
+        default_kwargs.update(kwargs)
         with localconverter(robjects.default_converter + pandas2ri.converter):
-            return KorAPClient.corpusStats(self, *args, **kwargs)
+            return KorAPClient.corpusStats(self, *args, **default_kwargs)
 
     def frequencyQuery(self, *args, **kwargs):
         """Query relative frequency of search term(s).
@@ -179,7 +181,7 @@
         - **verbose** - (default = `self.verbose`)
 
         Returns:
-            `KorAPQuery` | `pandas.DataFrame`
+            `KorAPQuery`
 
         Example:
             ```