Add end of file char as per DBF spec


The one small overlook here is that the end-of-file char is not output when you close the DBF. This has little impact since most software packages ignore this end of file char (it is part of the DBF spec) anyways.
There was a good reason why i didn't output it, but now i can't remember what it was. It had something to do with the forward only stream we support. But in any case, there should be a method to output the end-of-file char as per DBF spec. Once this is output the DBF class should lock writing.