Fix custom dim selection for string fields (#103)

* Fix custom dim selection for string fields

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

xpublish_wms/wms/get_map.py

@@ -242,13 +242,16 @@ def select_custom_dim(self, da: xr.DataArray) -> xr.DataArray:
         for dim, value in self.dim_selectors.items():
             if dim in da.coords:
                 dtype = da[dim].dtype
+                method = None
                 if "timedelta" in str(dtype):
                     value = pd.to_timedelta(value)
                 elif np.issubdtype(dtype, np.integer):
                     value = int(value)
+                    method = "nearest"
                 elif np.issubdtype(dtype, np.floating):
                     value = float(value)
-                da = da.sel({dim: value}, method="nearest")
+                    method = "nearest"
+                da = da.sel({dim: value}, method=method)
 
         # Squeeze single value dimensions
         da = da.squeeze()