Simplify updating new kwargs in NDCube.to_nddata

Author: DanRyanIrish

ndcube/ndcube.py

@@ -1476,11 +1476,12 @@ def to_nddata(self, *, nddata_type=NDData, **kwargs):
             or a class that behaves like one.  Default=`~astropy.nddata.NDData`.
 
         kwargs:
-            Attributes to change on output object. For example, to change the data on the
-            returned object compare to this instance, set a kwarg ``data=new_data``.
-            Note that kwargs set to ``None`` will not be passed to the constructor
-            of the ``nddata_type``. Therefore, if that attribute exists on the new type,
-            it will be given the default value defined by ``nddata_type``'s call signature.
+            Inputs to the ``nddata_type`` constructor that should differ from the values
+            stored in attributes of this instance. For example, to set different data values
+            on the returned object, set a kwarg ``data=new_data``, where ``new_data`` is an
+            an array of compatible shape and dtype.  Note that kwargs given by the user and
+            attributes on this instance that are not supported by the ``nddata_type``
+            constructor are ignored.
 
         Returns
         -------
@@ -1496,14 +1497,10 @@ def to_nddata(self, *, nddata_type=NDData, **kwargs):
 
         >>> nddata_without_coords = cube.to_nddata(wcs=None) # doctest: +SKIP
         """
-        # Build dictionary of new attribute values from this NDCube instance.
+        # Build dictionary of new attribute values from this NDCube instance
+        # and update with user-defined kwargs.
         new_kwargs = {key.strip("_"): value for key, value in self.__dict__.items()}
-        # Remove kwargs set to None and add kwargs that don't correspond to NDCube attrs.
-        for key, value in kwargs.items():
-            if value is None:
-                del new_kwargs[key]
-            else:
-                new_kwargs[key] = value
+        new_kwargs.update(kwargs)
         # Inspect call signature of new_nddata class and
         # remove unsupported items from new_kwargs.
         nddata_sig = inspect.signature(nddata_type).parameters.keys()