fix error Array to string conversion by implode meta keywords if its value is array

I have just downloaded and test with clean install, and then access ``example`` controller, and I got this error:

    A PHP Error was encountered  

    Severity: Notice  

    Message: Array to string conversion  

    Filename: themes/default.php  

    Line Number: 12  
    Array" />  

To solve it, I do this commit just check first, if meta keywords' value is array then implode it.