Revert "Revert "ENH #3715 specify arguments to _upsample""

This reverts commit 6f3fed3.

Author: nchmura4

pandas/core/generic.py

@@ -4019,7 +4019,7 @@ def asfreq(self, freq, method=None, how=None, normalize=False,
             Value to use for missing values, applied during upsampling (note
             this does not fill NaNs that already were present).
 
-            .. version added:: 0.20.0
+            .. versionadded:: 0.20.0
 
         Returns
         -------

pandas/tseries/resample.py

@@ -352,7 +352,7 @@ def transform(self, arg, *args, **kwargs):
     def _downsample(self, f):
         raise AbstractMethodError(self)
 
-    def _upsample(self, f, limit=None):
+    def _upsample(self, f, limit=None, fill_value=None):
         raise AbstractMethodError(self)
 
     def _gotitem(self, key, ndim, subset=None):
@@ -499,12 +499,21 @@ def interpolate(self, method='linear', axis=0, limit=None, inplace=False,
                                   limit_direction=limit_direction,
                                   downcast=downcast, **kwargs)
 
-    def asfreq(self, **kwargs):
+    def asfreq(self, fill_value=None):
         """
         return the values at the new freq,
-        essentially a reindex with (no filling)
+        essentially a reindex
+
+        Parameters
+        ----------
+        fill_value: scalar, optional
+            Value to use for missing values, applied during upsampling (note
+            this does not fill NaNs that already were present).
+
+            .. versionadded:: 0.20.0
+
         """
-        return self._upsample('asfreq', **kwargs)
+        return self._upsample('asfreq', fill_value=fill_value)
 
     def std(self, ddof=1, *args, **kwargs):
         """
@@ -710,6 +719,8 @@ def _upsample(self, method, limit=None, fill_value=None):
             'ffill', 'asfreq'} method for upsampling
         limit : int, default None
             Maximum size gap to fill when reindexing
+        fill_value : scalar, default None
+            Value to use for missing values
 
         See also
         --------
@@ -855,12 +866,14 @@ def _downsample(self, how, **kwargs):
             'Frequency {} cannot be resampled to {}, as they are not '
             'sub or super periods'.format(ax.freq, self.freq))
 
-    def _upsample(self, method, limit=None):
+    def _upsample(self, method, limit=None, fill_value=None):
         """
         method : string {'backfill', 'bfill', 'pad', 'ffill'}
             method for upsampling
         limit : int, default None
             Maximum size gap to fill when reindexing
+        fill_value : scalar, default None
+            Value to use for missing values
 
         See also
         --------