HTTP API: make sure virtual host limits are returned as a JSON object

when no limits are set.

This is the classic empty proplist JSON serialization
problem in a relatively new place.

(cherry picked from commit 1795306)

Author: michaelklishin

deps/rabbitmq_management/src/rabbit_mgmt_wm_limits.erl

@@ -45,13 +45,21 @@ limits(ReqData, Context) ->
         none ->
             User = Context#context.user,
             VisibleVhosts = rabbit_mgmt_util:list_visible_vhosts_names(User),
-            [ [{vhost, VHost}, {value, Value}]
-              || {VHost, Value} <- rabbit_vhost_limit:list(),
-                 lists:member(VHost, VisibleVhosts) ];
+            [
+                #{
+                    vhost => VHost,
+                    value => rabbit_data_coercion:to_map(Value)
+                } || {VHost, Value} <- rabbit_vhost_limit:list(), lists:member(VHost, VisibleVhosts)
+            ];
         VHost when is_binary(VHost) ->
             case rabbit_vhost_limit:list(VHost) of
                 []    -> [];
-                Value -> [[{vhost, VHost}, {value, Value}]]
+                Value -> [
+                    #{
+                        vhost => VHost,
+                        value => rabbit_data_coercion:to_map(Value)
+                    }
+                ]
             end
     end.
 %%--------------------------------------------------------------------

deps/rabbitmq_management/test/rabbit_mgmt_http_SUITE.erl

@@ -3411,13 +3411,14 @@ vhost_limits_list_test(Config) ->
     lists:map(
         fun(#{vhost := VHost, value := Val}) ->
             Param = [ {atom_to_binary(K, utf8),V} || {K,V} <- maps:to_list(Val) ],
+            ct:pal("Setting limits of virtual host '~ts' to ~tp", [VHost, Param]),
             ok = rabbit_ct_broker_helpers:set_parameter(Config, 0, VHost, <<"vhost-limits">>, <<"limits">>, Param)
         end,
         Expected),
 
-    Expected = http_get(Config, "/vhost-limits", ?OK),
-    Limits1 = http_get(Config, "/vhost-limits/limit_test_vhost_1", ?OK),
-    Limits2 = http_get(Config, "/vhost-limits/limit_test_vhost_2", ?OK),
+    ?assertEqual(lists:usort(Expected), lists:usort(http_get(Config, "/vhost-limits", ?OK))),
+    ?assertEqual(Limits1, http_get(Config, "/vhost-limits/limit_test_vhost_1", ?OK)),
+    ?assertEqual(Limits2, http_get(Config, "/vhost-limits/limit_test_vhost_2", ?OK)),
 
     NoVhostUser = <<"no_vhost_user">>,
     rabbit_ct_broker_helpers:add_user(Config, NoVhostUser),