Indexing fix: buildIndex() was leading to corrupted index values when more than one field was index

Hi Hugo, congrats for this great library, the way your approach secondary indexing in the context of localStorage is very smart.
I have found a small issue with indexes though: If more than one field is indexed, the buildIndex() function creates a corrupted value for indexes beyond the first one.

Example:
var db = new Database( { name: "test", indexedKeys: [ "shape", "color" ] );

After having inserted a few items and you change the "color" attribute of one of them the "test:color:*" records of localStorage become wrong.

The fix corrects this.

Cordially,

Mikael