Reintroduce BUrlResult and add BDataRequest

* 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.

headers/os/net/HttpRequest.h

@@ -47,7 +47,7 @@ public:
 									const ssize_t size = -1);
             void                AdoptHeaders(BHttpHeaders* const headers);
 
-			const BHttpResult&	Result() const;
+			const BUrlResult&	Result() const;
 			const char*			StatusString(status_t threadStatus) const;
 
 	static	bool				IsInformationalStatusCode(int16 code);