Chat request response format (#82)

* Added ResponseFormat to ChatCompletionRequest
Added Test for json mode
Corrected failing integration test

* Changed ResponseFormat to use a class rather than an anonymous object.

* Corrected integration tests

src/OpenAI.Net.Integration.Tests/ImageService_Variation.cs

@@ -32,7 +32,7 @@ public async Task Variation_Base64ToFileContent(bool isSuccess, HttpStatusCode s
                 o.Size = "256x256";
                 o.ResponseFormat = ImageResponseFormat.Base64;
                 o.Style = ImageStyleOptions.Vivid;
-                o.Quality = ImageQualityOptions.HighDefinition;
+                o.Quality = ImageQualityOptions.Standard;
                 o.Model = ImageModelOptions.Dalle2;
             });
 
@@ -47,7 +47,7 @@ public async Task Variation_Base64ToFileContent(bool isSuccess, HttpStatusCode s
             Assert.That(response.StatusCode, Is.EqualTo(statusCode));
             Assert.That(response.Result?.Data?.Count() == 1, Is.EqualTo(isSuccess), "Data is not mapped correctly");
             Assert.That(response.Result?.Data?[0].Url?.Contains("https://"), isSuccess ? Is.EqualTo(isSuccess) : Is.EqualTo(null), "Choice text not set");
-            Assert.That(response.ErrorResponse?.Error?.Message?.Contains("is not one of ['256x256', '512x512', '1024x1024']"), isSuccess ? Is.EqualTo(null) : Is.EqualTo(true), "Error message not returned");
+            Assert.That(response.ErrorResponse?.Error?.Message, isSuccess ? Is.EqualTo(null) : Contains.Substring("is not one of ['256x256', '512x512', '1024x1024']"), "Error message not returned");
         }
 
         [TestCase(true, HttpStatusCode.OK, "256x256", TestName = "Variation_FileInfoFileContent_When_Success")]