Fix URL encoding of HttpUtility.ParseQueryString's ToString method (#98263)

* Fix URL encoding of `HttpUtility.ParseQueryString`'s `ToString` method

* Add test

Author: RenderMichael

src/libraries/System.Web.HttpUtility/src/System/Web/HttpUtility.cs

@@ -67,7 +67,7 @@ public override string ToString()
                         {
                             if (!string.IsNullOrEmpty(key))
                             {
-                                sb.Append(key).Append('=');
+                                sb.Append(UrlEncode(key)).Append('=');
                             }
                             sb.Append(UrlEncode(value)).Append('&');
                         }

src/libraries/System.Web.HttpUtility/tests/HttpUtility/HttpUtilityTest.cs

@@ -792,6 +792,7 @@ public void UrlPathEncode(string decoded, string encoded)
         [InlineData("foo&bar")]
         [InlineData("foo&name=bar")]
         [InlineData("name=bar&foo&foo")]
+        [InlineData("_return_fields%2b=extattrs&name%3a=somename.somedomain.local")]
         public void ParseAndToStringMaintainAllKeyValuePairs(string input)
         {
             var values = HttpUtility.ParseQueryString(input);