Support for encoding option in fs.readFile and fs.readFileSync

A very small fix to make the encoding option work in fs.readFile and fs.readFileSync. These calls will all now return a string:

    fs.readFile(path, 'utf8', callback);
    fs.readFile(path, {encoding: 'utf8'}, callback);
    fs.readFileSync(path, 'utf8');
    fs.readFileSync(path, {encoding: 'utf8'});

I added a couple of tests, and the existing tests all pass.