Support multiple value while identify image.

When metadata has multiple value like keywords parser(./lib/getters.js)
just take the last because parser override the previous value.
Instead of override this commit cast it to array.

lib/getters.js

@@ -228,7 +228,20 @@ module.exports = function (gm) {
         level = indent;
 
         if (val) {
-          o[key] = val;
+          // if previous key was exist and we got the same key
+          // cast it to an array.
+          if(o.hasOwnProperty(key)){
+            // cast it to an array and dont forget the previous value
+            if(!Array.isArray(o[key])){
+              var tmp = o[key];
+              o[key] = [tmp];
+            }
+
+            // set value
+            o[key].push(val);
+          } else {
+            o[key] = val;
+          }
 
           if (key in helper) {
             helper[key](o, val);
@@ -330,3 +343,4 @@ module.exports = function (gm) {
     }
   };
 }
+