[BUG] dask_cudf and cudf not able to cast to str dtype if using dict

[kshitizgupta21]

Describe the bug
dask cudf and cudf error out if trying to pass in dict for casting to string dtype in .astype

Steps/Code to reproduce bug

import dask_cudf
import cudf

# cast col_a to string and col_b to int
df = cudf.DataFrame({'col_a' : [1, 2, 4],
                     'col_b' : ['11', '22', '44']
                    })


ddf = dask_cudf.from_cudf(df, npartitions=2)

ddf.astype({'col_a': str,
            'col_b' : int})
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-111-523edc1889ba> in <module>
----> 1 ddf.astype({'col_a': 'str', 'col_b' : int})

/opt/conda/lib/python3.8/site-packages/dask/dataframe/core.py in astype(self, dtype)
   2666             meta = self._meta_nonempty.astype(dtype)
   2667         else:
-> 2668             meta = self._meta.astype(dtype)
   2669         if hasattr(dtype, "items"):
   2670             set_unknown = [

/opt/conda/lib/python3.8/site-packages/cudf/core/dataframe.py in astype(self, dtype, copy, errors, **kwargs)
   1208             for col_name in current_cols:
   1209                 if col_name in dtype:
-> 1210                     result._data[col_name] = self._data[col_name].astype(
   1211                         dtype=dtype[col_name],
   1212                         errors=errors,

/opt/conda/lib/python3.8/site-packages/cudf/core/column/column.py in astype(self, dtype, **kwargs)
   1040             str,
   1041         }:
-> 1042             return self.as_string_column(dtype, **kwargs)
   1043         elif is_list_dtype(dtype):
   1044             if not self.dtype == dtype:

TypeError: as_string_column() got an unexpected keyword argument 'errors'

df.astype({'col_a': str,
           'col_b' : int})
Same error as above

Expected behavior
Expected the conversion to go through.

Environment details
cuDF v.0.19.1 run through NVTabular Merlin v0.5.3 NGC Merlin Container