Take care with str vs bytes in the implementation

Also replace the intentional BAD_REQUEST with GONE for this case.

src/allmydata/testing/web.py

@@ -6,22 +6,13 @@
 # This file is part of Tahoe-LAFS.
 #
 # See the docs/about.rst file for licensing information.
-"""Test-helpers for clients that use the WebUI.
-
-Ported to Python 3.
 
 """
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
-from future.utils import PY2
-if PY2:
-    from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
-
+Test-helpers for clients that use the WebUI.
+"""
 
 import hashlib
+from typing import Dict
 
 import attr
 
@@ -147,7 +138,7 @@ class _FakeTahoeUriHandler(Resource, object):
 
     isLeaf = True
 
-    data = attr.ib(default=attr.Factory(dict))
+    data: Dict[bytes, bytes] = attr.ib(default=attr.Factory(dict))
     capability_generators = attr.ib(default=attr.Factory(dict))
 
     def _generate_capability(self, kind):
@@ -209,7 +200,7 @@ class _FakeTahoeUriHandler(Resource, object):
         capability = None
         for arg, value in uri.query:
             if arg == u"uri":
-                capability = value
+                capability = value.encode("utf-8")
         # it's legal to use the form "/uri/<capability>"
         if capability is None and request.postpath and request.postpath[0]:
             capability = request.postpath[0]
@@ -221,10 +212,9 @@ class _FakeTahoeUriHandler(Resource, object):
 
         # the user gave us a capability; if our Grid doesn't have any
         # data for it, that's an error.
-        capability = capability.encode('ascii')
         if capability not in self.data:
-            request.setResponseCode(http.BAD_REQUEST)
-            return u"No data for '{}'".format(capability.decode('ascii'))
+            request.setResponseCode(http.GONE)
+            return u"No data for '{}'".format(capability.decode('ascii')).encode("utf-8")
 
         return self.data[capability]