allow access to metadata collection attributes

follow up to https://github.com/galaxyproject/galaxy/pull/8599

in the planemo tests in the line https://github.com/bernt-matthias/galaxy/blob/1882aa641cdde1c51c077fd983f2241971308377/lib/galaxy/tools/parameters/dynamic_options.py#L205

r.metadata is sometimes a MetadataWrapper and sometimes a MetadataCollection

in the former case acccess to the `element_is_set` function defined in MetadataCollection is not possible.

lib/galaxy/tools/wrappers.py

@@ -214,6 +214,9 @@ class DatasetFilenameWrapper(ToolParameterValueWrapper):
             self.compute_environment = compute_environment
 
         def __getattr__(self, name):
+            if hasattr(self.metadata, name):
+                return getattr(self.metadata, name)
+
             rval = self.metadata.get(name, None)
             if name in self.metadata.spec:
                 if rval is None: