Bug 1633309 [wpt PR 23257] - [wptserve] Fix an exception handler in stash, a=testonly

Automatic update from web-platform-tests
[wptserve] Fix an exception handler in stash

internal_key is a tuple whose items are guaranteed to be strings.
It should be used as the key directly.
--

wpt-commits: d2025b24b0b1f4d2e69e83cf83e60c021d9f7b42
wpt-pr: 23257

testing/web-platform/tests/tools/wptserve/wptserve/stash.py

@@ -162,7 +162,7 @@ class Stash(object):
         if internal_key in self.data:
             raise StashError("Tried to overwrite existing shared stash value "
                              "for key %s (old value was %s, new value is %s)" %
-                             (internal_key, self.data[str(internal_key)], value))
+                             (internal_key, self.data[internal_key], value))
         else:
             self.data[internal_key] = value