WriteConsole() callback is missing the last character

Playing around with RINSIDE_CALLBACKS, I noticed that the last character of each line was missing in WriteConsole(). It's a simple one-liner fix.

Since there are no testcases in RInside, I added an example instead.

Note that currently both callback-examples will leak the Callbacks object. I'm not sure if Rinside is supposed to take ownership (in which case it's missing a delete somewhere) or not (in which case the examples need to be adjusted).
Should that not be `assign(buf, buf + len)` ?