[RFC] - King can take his own figure BUG FIX

This commit is fixing a bug in the ChessGame.php file. The king was able to take his own figures, due to a bug in the validation ChessGame::_convertSquareToSAN(). The possible moves were not checked for the king and this commit fixes that.

Related ticket(s):
https://chess.codebasehq.com/projects/development/tickets/3946
https://chess.codebasehq.com/projects/development/tickets/3961

Todo:

- [x] - fix bug :P

@BiviaBen 
@kalifg 
@marcosdsanchez