fix(HTTPUnauthorized): Move scheme into kwargs

This patch moves scheme from being it's own param into kwargs, to
discourage using it as a positional arg. This will avoid subtle
bugs in the future that may arise as the signature for the
HTTPError constructor changes over time.

BREAKING CHANGE: The 'scheme' argument to HTTPUnauthorized can no
    longer be passed positionally; it must be a named argument.

falcon/exceptions.py

@@ -50,19 +50,9 @@ class HTTPUnauthorized(HTTPError):
     Use when authentication is required, and the provided credentials are
     not valid, or no credentials were provided in the first place.
 
-    Args:
-        title: Human-friendly error title
-        description: Human-friendly description of the error, along with a
-            helpful suggestion or two.
-        scheme: Authentication scheme to use as the value of the
-            WWW-Authenticate header in the response (default None).
-
-    The remaining (optional) args are the same as for HTTPError.
-
-
     """
 
-    def __init__(self, title, description, scheme=None, **kwargs):
+    def __init__(self, title, description, **kwargs):
         """Initialize
 
         Args:
@@ -77,6 +67,8 @@ class HTTPUnauthorized(HTTPError):
 
         """
         headers = kwargs.setdefault('headers', {})
+
+        scheme = kwargs.pop('scheme', None)
         if scheme is not None:
             headers['WWW-Authenticate'] = scheme
 

falcon/tests/test_httperror.py

@@ -67,7 +67,7 @@ class UnauthorizedResource:
     def on_get(self, req, resp):
         raise falcon.HTTPUnauthorized('Authentication Required',
                                       'Missing or invalid token header.',
-                                      'Token')
+                                      scheme='Token; UUID')
 
 
 class UnauthorizedResourceSchemaless:
@@ -259,7 +259,8 @@ class TestHTTPError(testing.TestBase):
         self.simulate_request('/401')
 
         self.assertEqual(self.srmock.status, falcon.HTTP_401)
-        self.assertIn(('WWW-Authenticate', 'Token'), self.srmock.headers)
+        self.assertIn(('WWW-Authenticate', 'Token; UUID'),
+                      self.srmock.headers)
 
     def test_401_schemaless(self):
         self.api.add_route('/401', UnauthorizedResourceSchemaless())