Check if session data array is for checkboxes

As reported in
#693
the session data can get overwritten. This happened if
there is an array in the session data as the logic takes
this to be checkboxes data and stores it, overwriting
the existing data.

lib/utils.js

@@ -236,11 +236,8 @@ var storeData = function (input, data) {
     }
 
     // Remove _unchecked from arrays of checkboxes
-    if (Array.isArray(val)) {
-      var index = val.indexOf('_unchecked')
-      if (index !== -1) {
-        val.splice(index, 1)
-      }
+    if (Array.isArray(val) && val.indexOf('_unchecked') !== -1) {
+      val.splice(val.indexOf('_unchecked'), 1)
     } else if (typeof val === 'object') {
       // Store nested objects that aren't arrays
       if (typeof data[i] !== 'object') {