[http_ng] Fix regression

In PR #1503 we return nil and error in case of an error, however some
code paths do not handle the error but instead check the error field of
the response object and lead to unexpected behavior.

With this PR, we will return a response object with the error field set.

Author: tkan145

gateway/src/resty/http_ng/backend/resty.lua

@@ -37,7 +37,7 @@ backend.send = function(_, request)
   local httpc, err = http_proxy.new(request)
 
   if err then
-    return nil, err
+    return response.error(request, err)
   end
 
   if httpc then

spec/resty/http_ng/backend/resty_spec.lua

@@ -37,8 +37,11 @@ describe('resty backend', function()
       local req = { method = method, url = 'http://0.0.0.0:0/' }
       local response, err = backend:send(req)
 
-      assert.falsy(response)
-      assert.equal("connection refused", err)
+      assert.falsy(err)
+      assert.truthy(response.error)
+      assert.equal("connection refused", response.error)
+      assert.falsy(response.ok)
+      assert.same(req, response.request)
     end)
 
     context('http proxy is set', function()

spec/resty/http_ng_spec.lua

@@ -224,18 +224,18 @@ describe('http_ng', function()
   end)
 
   describe('when there is error #network', function()
-    local response, err
+    local response
     before_each(function()
       http = http_ng.new{ backend = resty_backend }
-      response, err = http.get('http://127.0.0.1:1')
+      response = http.get('http://127.0.0.1:1')
     end)
 
     it('is not ok', function()
-      assert.falsy(response)
+      assert.equal(false, response.ok)
     end)
 
     it('has error', function()
-      assert.equal("connection refused", err)
+      assert.equal("connection refused", response.error)
     end)
   end)