Improve Request overrides on ContinueAsync (#617)

* Improve Request overrides on ContinueAsync

* cr and better test

Author: kblok

lib/PuppeteerSharp.Tests/Issues/Issue0616.cs

@@ -0,0 +1,50 @@
+using Microsoft.AspNetCore.Http;
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Net;
+using System.Net.Http;
+using System.Text;
+using System.Threading.Tasks;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace PuppeteerSharp.Tests.Issues
+{
+    [Collection("PuppeteerLoaderFixture collection")]
+    public class Issue0616 : PuppeteerPageBaseTest
+    {
+        public Issue0616(ITestOutputHelper output) : base(output)
+        {
+        }
+
+        [Fact]
+        public async Task ShouldBeAbleToChangeToPost()
+        {
+            await Page.SetRequestInterceptionAsync(true);
+            Page.Request += async (sender, e) =>
+            {
+                var payload = new Payload()
+                {
+                    Method = HttpMethod.Post,
+                    PostData = "foo"
+                };
+                await e.Request.ContinueAsync(payload);
+            };
+
+            Server.SetRoute("/grid.html", async (context) =>
+            {
+                using (var reader = new StreamReader(context.Request.Body, Encoding.UTF8))
+                {
+                    var request = await reader.ReadToEndAsync();
+                    await context.Response.WriteAsync(request);
+                }
+            });
+            var response = await Page.GoToAsync(TestConstants.ServerUrl + "/grid.html");
+
+            Assert.Equal(TestConstants.ServerUrl + "/grid.html", response.Url);
+            Assert.Equal("foo", await response.TextAsync());
+        }
+    }
+}

lib/PuppeteerSharp/Payload.cs

@@ -13,7 +13,10 @@ namespace PuppeteerSharp
     /// </summary>
     public class Payload
     {
-        internal Payload()
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PuppeteerSharp.Payload"/> class.
+        /// </summary>
+        public Payload()
         {
             Headers = new Dictionary<string, object>();
         }

lib/PuppeteerSharp/Request.cs

@@ -180,15 +180,15 @@ public async Task ContinueAsync(Payload overrides = null)
 
                 if (overrides?.Method != null)
                 {
-                    requestData["method"] = overrides.Method;
+                    requestData["method"] = overrides.Method.ToString();
                 }
 
                 if (overrides?.PostData != null)
                 {
                     requestData["postData"] = overrides.PostData;
                 }
 
-                if (overrides?.Headers != null)
+                if (overrides?.Headers?.Count > 0)
                 {
                     requestData["headers"] = overrides.Headers;
                 }