FormDataMatcher does not decode + into space correctly

[jozefizso]

When using MockHttpMessageHandler.WithFormData() to match form data that use spaces, matching does not work correctly because + sign in the data is not unescaped correctly to space.

var data = new Dictionary<string, string>() { {"param", "Parameter with space" } };
var dataForHttpClient = new FormUrlEncodedContent(data);

// query string produced by FormUrlEncodedContent.Encode():
// param=Parameter+with+space

var httpMock = new MockHttpMessageHandler();
httpMock.Expect(HttpMethod.Post, "https://server.org/api/command")
.WithFormData(data);

// will throw exception because FormDataMatcher will not convert + to back to space
httpMock.VerifyNoOutstandingExpectation();

// FormUrlEncodedContent from HttpClient library
public class FormUrlEncodedContent : ByteArrayContent
{
    private static string Encode(string data)
    {
      if (string.IsNullOrEmpty(data))
        return string.Empty;
      return Uri.EscapeDataString(data).Replace("%20", "+");
    }
}

FormDataMatcher is using QueryStringMatcher.ParseQueryString() to decode posted data. That method uses only Uri.UnescapeDataString() which does not follow URL spec and does not convert + back to space.

More info on URLs and URIs in .NET: Don't use .NET System.Uri.UnescapeDataString in URL Decoding