Fix replication ID parsing in URL paths

Previously users had to URL encode replication IDs when using
`_scheduler/jobs/<job_id>` endpoint because Mochiweb incorrectly decoded the
`+` character from URL path. So users were forced to encode so that the
replicator would correctly receive a `+` after Mochiweb parsing.

`+` is decoded as ` ` (space) probably because in query strings that's a valid
application/x-www-form-urlencoded encoding, but that decoding is not meant for
decoding URL paths, only query strings.

Notice RFC 3986 https://tools.ietf.org/html/rfc3986#section-2.2

`+` is a `sub-delim` (term from RFC)  and in the path component it can be used
unquoted as a delimiter.

https://tools.ietf.org/html/rfc3986#section-3.3

Indeed, the replication ID is a compound ID and `+` is a valid delimiter
which separates the base part from the extensions.

For more details see also:

perwendel/spark#490

https://www.w3.org/TR/html401/interact/forms.html#h-17.13.4.1

Fixes apache#825

src/couch_replicator/src/couch_replicator_ids.erl

@@ -78,7 +78,11 @@ replication_id(#rep{user_ctx = UserCtx} = Rep, 1) ->
 -spec convert([_] | binary() | {string(), string()}) -> {string(), string()}.
 convert(Id) when is_list(Id) ->
     convert(?l2b(Id));
-convert(Id) when is_binary(Id) ->
+convert(Id0) when is_binary(Id0) ->
+    % Spaces can result from mochiweb incorrectly unquoting + characters from
+    % the URL path. So undo the incorrect parsing here to avoid forcing
+    % users to url encode + characters.
+    Id = binary:replace(Id0, <<" ">>, <<"+">>, [global]),
     lists:splitwith(fun(Char) -> Char =/= $+ end, ?b2l(Id));
 convert({BaseId, Ext} = Id) when is_list(BaseId), is_list(Ext) ->
     Id.
@@ -222,6 +226,16 @@ get_non_default_port(_Schema, Port) ->
 
 -include_lib("eunit/include/eunit.hrl").
 
+
+replication_id_convert_test_() ->
+    [?_assertEqual(Expected, convert(Id)) || {Expected, Id} <- [
+        {{"abc", ""}, "abc"},
+        {{"abc", ""}, <<"abc">>},
+        {{"abc", "+x+y"}, <<"abc+x+y">>},
+        {{"abc", "+x+y"}, {"abc", "+x+y"}},
+        {{"abc", "+x+y"}, <<"abc x y">>}
+    ]].
+
 http_v4_endpoint_test_() ->
     [?_assertMatch({remote, User, Host, Port, Path, HeadersNoAuth, undefined},
         get_v4_endpoint(nil, #httpdb{url = Url, headers = Headers})) ||