fix(api) consumers endpoint filtering by custom id

This issue was reported by @JoarSvensson on issue #4402.

kong/api/routes/consumers.lua

@@ -14,8 +14,8 @@ return {
 
       -- Search by custom_id: /consumers?custom_id=xxx
       if args.custom_id then
-        self.params.consumers = args.custom_id
-        local consumer, _, err_t = endpoints.select_entity(self, db, db.consumers.schema, "select_by_custom_id")
+        local opts = endpoints.extract_options(args, db.consumers.schema, "select")
+        local consumer, _, err_t = db.consumers:select_by_custom_id(args.custom_id, opts)
         if err_t then
           return endpoints.handle_error(err_t)
         end

spec/02-integration/04-admin_api/03-consumers_routes_spec.lua

@@ -255,6 +255,17 @@ describe("Admin API (#" .. strategy .. "): ", function()
         local body = assert.res_status(200, res)
         local json = cjson.decode(body)
 
+        assert.equal(1, #json.data)
+        assert.same(c, json.data[1])
+      end)
+      it("allows filtering by uuid-like custom_id", function()
+        local custom_id = utils.uuid()
+        local c = bp.consumers:insert({ custom_id = custom_id }, { nulls = true })
+
+        local res = client:get("/consumers?custom_id=" .. custom_id)
+        local body = assert.res_status(200, res)
+        local json = cjson.decode(body)
+
         assert.equal(1, #json.data)
         assert.same(c, json.data[1])
       end)