Change fs.truncate to act like truncate(2) and extend the file, also.

The standard truncate also extends a file when given a length greater than the current file length. It would be nice for vert.x to emulate this behavior.

    TRUNCATE(2)                 BSD System Calls Manual                TRUNCATE(2)
    
    NAME
         ftruncate, truncate -- truncate or extend a file to a specified length