* BUrlResult is back, with ContentType and Length methods.
* BHttpResult subclasses it and use HTTP header fields to implement
those
* Introduce BDataRequest for "data" URIs. These embed the data inside
the URI, either as plaintext or base64 encoded.
* Remove the fRawData field, as handling it is too complicated (it's
not easy to have proper copy semantics on a BDataIO) and it's not used
anyway, as the listener DataReceived call is enough to get the data and
handle it.
* All the remaining fields are HTTP-only, so rename the class to
HttpResult and attach it to HttpRequest instead of UrlRequest.
