event/BufferedSocket: pass writable pointer to OnSocketInput()

Remove the const_cast from HttpdClient.cxx, and avoid one allocation
in ClientRead.cxx.

src/ClientInternal.hxx

@@ -111,8 +111,7 @@ public:
 
 private:
 	/* virtual methods from class BufferedSocket */
-	virtual InputResult OnSocketInput(const void *data,
-					  size_t length) override;
+	virtual InputResult OnSocketInput(void *data, size_t length) override;
 	virtual void OnSocketError(Error &&error) override;
 	virtual void OnSocketClosed() override;