`HttpException.statusCode` may be null

[rfc2822]

Since #83, the statusCode of HttpException may be null.

However a HttpException is by its design created on a HTTP error condition with a status code, and DAVx5 relies on that status code. Wrapping everything into if (statusCode != null) is inconvenient, especially because the statusCode is var.

So there are still two problems (the first is more important):

• HttpException should always have a non-null status code.
• Immutable things should be immutable.