fs: improve error message for invalid flag

Flags on fs.open and others can be passed as strings or int.
Previously, if passing anything other than string or int,
the error message would only say that flags must be an int.

PR-URL: https://github.com/nodejs/node/pull/5590
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

lib/fs.js

@@ -536,8 +536,8 @@ fs.readFileSync = function(path, options) {
 
 // Used by binding.open and friends
 function stringToFlags(flag) {
-  // Only mess with strings
-  if (typeof flag !== 'string') {
+  // Return early if it's a number
+  if (typeof flag === 'number') {
     return flag;
   }