Update schedule.js

mapDayName - I received null error here when testing the current version.  I changed the array order, and restricted the assignment to occur only with parameter values in a given range (0-6).  Function returns empty string instead of null if the parameter does not match an array element.
Merge pull request #2 from andersonk17474/patch-1

Make sure mapDayName's array of days is mirroring Date().getDay() and add range restriction.