Support auth cookies with : characters

The parts of a couchdb authentication cookie are separated by
colons. One of these parts can contain colons and, more rarely, runs
of colons. The string:tokens function silently drops any empty token,
thus giving a spurious failure for valid input. The fix changes this
mechanism to one that losslessly decodes this part.

COUCHDB-1607

src/couchdb/couch_httpd_auth.erl

@@ -160,7 +160,7 @@ cookie_authentication_handler(#httpd{mochi_req=MochiReq}=Req) ->
     Cookie ->
         [User, TimeStr | HashParts] = try
             AuthSession = couch_util:decodeBase64Url(Cookie),
-            [_A, _B | _Cs] = string:tokens(?b2l(AuthSession), ":")
+            [_A, _B | _Cs] = re:split(?b2l(AuthSession), ":", [{return, list}])
         catch
             _:_Error ->
                 Reason = <<"Malformed AuthSession cookie. Please clear your cookies.">>,