(fix) correlation-id ensure plugin header not nil

Fix clashing plugins issue reported in Kong#3924

kong/plugins/correlation-id/handler.lua

@@ -80,7 +80,7 @@ end
 function CorrelationIdHandler:header_filter(conf)
   CorrelationIdHandler.super.header_filter(self)
 
-  if conf.echo_downstream then
+  if conf.echo_downstream and kong.ctx.plugin.correlationid_header_value then
     kong.response.set_header(conf.header_name, kong.ctx.plugin.correlationid_header_value)
   end
 end

spec/03-plugins/12-correlation-id/01-access_spec.lua

@@ -29,6 +29,10 @@ for _, strategy in helpers.each_strategy() do
       local route4 = bp.routes:insert {
         hosts = { "correlation-tracker.com" },
       }
+
+      local route5 = bp.routes:insert {
+        hosts = { "correlation5.com" },
+      }
 
       bp.plugins:insert {
         name     = "correlation-id",
@@ -59,6 +63,24 @@ for _, strategy in helpers.each_strategy() do
           generator = "tracker",
         },
       }
+
+      bp.plugins:insert {
+        name     = "correlation-id",
+        route = { id = route5.id },
+        config   = {
+          generator       = "uuid",
+          echo_downstream = true,
+        },
+      }
+
+      bp.plugins:insert {
+        name     = "request-termination",
+        route = { id = route5.id },
+        config   = {
+          status_code = 200,
+          message     = "Success",
+        },
+      }
 
       assert(helpers.start_kong({
         database   = strategy,
@@ -114,7 +136,7 @@ for _, strategy in helpers.each_strategy() do
       end)
     end)
 
-    describe("uuid genetator", function()
+    describe("uuid generator", function()
       it("generates a unique UUID for every request", function()
         local res = assert(proxy_client:send {
           method  = "GET",
@@ -228,5 +250,18 @@ for _, strategy in helpers.each_strategy() do
       local id   = json.headers["kong-request-id"]
       assert.equal("foobar", id)
     end)
+
+    it("successfully executes with echo_downstream and request-termination enabled", function()
+      local res = assert(proxy_client:send {
+        method  = "GET",
+        path    = "/request",
+        headers = {
+          ["Host"]            = "correlation5.com"
+        }
+      })
+      local body = assert.res_status(200, res)
+      local json = cjson.decode(body)
+      assert.same({ message = "Success" }, json)
+    end)
   end)
-end
+end