Edited Issue: Key to the HttpError dictionary shall be public [187]

Following keys are private:
private const string MessageKey = "Message";
private const string MessageDetailKey = "MessageDetail";
private const string ModelStateKey = "ModelState";
private const string ExceptionMessageKey = "ExceptionMessage";
private const string ExceptionTypeKey = "ExceptionType";
private const string StackTraceKey = "StackTrace";
private const string InnerExceptionKey = "InnerException";

It makes it difficult for client to extract error information directly. If the string is hard coded and in the future the keys are changed, then maintaining cost is introduced.