Update NaN block size tests for threshold_local function (#289)

* Do not expect ValueError when gaussian block_size is Nan

Improvements elsewhere mean the ValueError is no longer raised.

* cupy.nan is float type, must explicitly specify like kwarg to match cupy array type

dask_image/ndfilters/_threshold.py

@@ -80,7 +80,7 @@ def threshold_local(image, block_size, method='gaussian', offset=0,
                                                mode=mode, cval=cval)
     elif method == 'gaussian':
         if param is None:
-            sigma = (da.asarray(block_size) - 1) / 6.0
+            sigma = (da.asarray(block_size, like=image._meta) - 1) / 6.0
         else:
             sigma = param
         thresh_image = _gaussian.gaussian_filter(image, sigma, mode=mode,

tests/test_dask_image/test_ndfilters/test_cupy_threshold.py

@@ -106,7 +106,6 @@ def setup(self):
              [4, 5, 1, 0, 0]], dtype=int), chunks=(5, 5))
 
     @pytest.mark.parametrize("method, block_size, error_type", [
-        ('gaussian', cupy.nan, ValueError),
         ('median', cupy.nan, TypeError),
     ])
     def test_nan_blocksize(self, method, block_size, error_type):