Fixed bug when nested POST variables contained excessive characters

We faced a pity situation when the form

```
<form ...>
<input name="question[options[a]]" />
</form>
```

being submit returned the object

```
{
  question: {
    options: {
      'a]': 'the value'
    }
  }
}
```

Found the bug when excessive elements were merged into the object, fixed simply removing them from the parts array being merged.

**Note: this is a major-priority bugfix as it may break much functionality!**