Description

warn_on_dtype has no effect when input is a pandas DataFrame

Steps/Code to Reproduce

from sklearn.utils.validation import check_array
import pandas as pd
df = pd.DataFrame([[1, 2, 3], [2, 3, 4]], dtype=object)
checked = check_array(df, warn_on_dtype=True)

Expected result:

DataConversionWarning: Data with input dtype object was converted to float64.

Actual Results

No warning is thrown

Versions

Linux-4.4.0-116-generic-x86_64-with-debian-stretch-sid
Python 3.6.3 |Anaconda, Inc.| (default, Nov 3 2017, 19:19:16)
[GCC 7.2.0]
NumPy 1.13.1
SciPy 0.19.1
Scikit-Learn 0.20.dev0
Pandas 0.21.0