c# - MVC Action Filter in IIS 7.5+ modifying response

Question

I have a MVC action filter that I use to validate my models that is giving me different responses on different versions of IIS. Here is the code for the action filter.

    public class ValidateModelAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        var viewData = filterContext.Controller.ViewData;
        var modelStateErrors = filterContext.Controller.ViewData.ModelState.Keys.SelectMany(key => filterContext.Controller.ViewData.ModelState[key].Errors);
        string errorMessage;

        if (modelStateErrors.Count() > 0)
        {
            ModelError error = modelStateErrors.FirstOrDefault();

            if (error.ErrorMessage.IsNotNullOrEmpty())
            {
                errorMessage = error.ErrorMessage;
            }
            else if (error.Exception != null)
            {
                errorMessage = error.Exception.Message;
            }
            else
            {
                errorMessage = Messages.UNKNOWN_VALIDATION_ERROR;
            }
        }
        else
        {
            errorMessage = Messages.UNKNOWN_VALIDATION_ERROR;
        }

        if (!viewData.ModelState.IsValid)
        {
            if (filterContext.HttpContext.Request.IsAjaxRequest())
            {
                filterContext.HttpContext.Response.StatusCode = (int)HttpStatusCode.BadRequest;
                filterContext.Result = new ContentResult { Content = errorMessage, ContentType = "text/plain" };
            }
            else
            {
                var controller = filterContext.Controller as WMSControllerBase;
                if (controller != null)
                {
                    controller.DisplayWarningMessage(errorMessage);
                }

                foreach (var param in filterContext.ActionParameters)
                {
                    if (param.Key.ToLower() == "model" || param.Value is ViewModelBase)
                    {
                        viewData.Model = param.Value;
                        break;
                    }
                }

                if (viewData.Model is ViewModelBase)
                {
                    (viewData.Model as ViewModelBase).RebuildModel();
                }

                filterContext.Result = new ViewResult
                {
                    ViewData = viewData
                };
            }
        }

        base.OnActionExecuting(filterContext);
    }
}

On an IIS 7.0 server the action filter works as intended and will produce responses like the following for AJAX calls.

HTTP/1.1 400 Bad Request
Cache-Control: private
Content-Type: text/plain; charset=utf-8
Server: Microsoft-IIS/7.0
X-AspNetMvc-Version: 4.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Tue, 08 Oct 2013 21:40:36 GMT
Content-Length: 47

The error message I want the end user to see.

However running on IIS 7.5 or later I get responses like the following. Apparently IIS or MVC decides to replace my response body with "Bad Request" and sets the type to text/html. Wondering what is going here, why would it modify the response like this?

HTTP/1.1 400 Bad Request
Cache-Control: private
Content-Type: text/html
Server: Microsoft-IIS/7.5
X-AspNetMvc-Version: 4.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Tue, 08 Oct 2013 21:50:02 GMT
Content-Length: 11

Bad Request