fix http response parsing

The first issue was that the first headerline can have more than three parts after splitting with `' '` (space char). e.g. `HTTP/1.1 404 Not Found`.

The second issue was that the `ResponseBase` class automatically sets default headers for `Server`, `Date` and `Content-Type` and since the `HeaderCollection` does not allow multiple headers with the same name I had to make the `ResponseBase` constructor with the `int statusCode` argument internal and remove the header initialization. Which would be a breaking change...