Fixed mean (#129)

* Fixed mean with numeric_only=False

* Fixed mean returning the wrong value

* resolve comments

modin/data_management/data_manager.py

@@ -929,8 +929,8 @@ def mean(self, **kwargs):
         """
         # Pandas default is 0 (though not mentioned in docs)
         axis = kwargs.get("axis", 0)
-        func = self._prepare_method(pandas.DataFrame.mean, **kwargs)
-        return self.full_reduce(axis, func, numeric_only=True)
+        kwargs["numeric_only"] = True
+        return self.sum(**kwargs).divide(self.count(axis=axis, numeric_only=True))
 
     def min(self, **kwargs):
         """Returns the minimum from each column or row.

modin/pandas/dataframe.py

@@ -2168,6 +2168,8 @@ def mean(self, axis=None, skipna=None, level=None, numeric_only=None, **kwargs):
             The mean of the DataFrame. (Pandas series)
         """
         axis = pandas.DataFrame()._get_axis_number(axis) if axis is not None else 0
+        if numeric_only is not None and not numeric_only:
+            self._validate_dtypes(numeric_only=True)
         return self._data_manager.mean(
             axis=axis, skipna=skipna, level=level, numeric_only=numeric_only, **kwargs
         )