Update FormTypeParser.php

only [] isn't good enough because 

if form type has more that 1 field as example:

ROOT
 - FIELD
 - SUB
   -- SUB_FIELD
   -- SUB_FIELD_2

It will be 
root[field]
root[sub][][sub_field]
root[sub][][sub_field_2]

And it will be submitted as 2 collections instead of 1 
And if we will force to use equal key for array it will be 1 collection as supposed