minor changes to avoid convertion type warnings from sklearn transformers

Author: arielrossanigo

README.rst

@@ -57,7 +57,7 @@ Normally you'll read the data from a file, but for demonstration purposes we'll
 
     >>> data = pd.DataFrame({'pet':      ['cat', 'dog', 'dog', 'fish', 'cat', 'dog', 'cat', 'fish'],
     ...                      'children': [4., 6, 3, 3, 2, 3, 5, 4],
-    ...                      'salary':   [90, 24, 44, 27, 32, 59, 36, 27]})
+    ...                      'salary':   [90., 24, 44, 27, 32, 59, 36, 27]})
 
 Transformation Mapping
 ----------------------

tests/test_dataframe_mapper.py

@@ -625,7 +625,8 @@ def test_list_transformers():
     Specifying a list of transformers applies them sequentially to the
     selected column.
     """
-    dataframe = pd.DataFrame({"a": [1, np.nan, 3], "b": [1, 5, 7]})
+    dataframe = pd.DataFrame({"a": [1, np.nan, 3], "b": [1, 5, 7]},
+                             dtype=np.float64)
 
     mapper = DataFrameMapper([
         (["a"], [Imputer(), StandardScaler()]),