box.space[x].index[x].alter does not change index parts

[Roman Tsisyk]

localhost> space = box.schema.create_space('tweedledum', { id = 0 })
---
...
localhost> space:create_index('primary', 'hash', { parts = { 0, 'num' }})
---
...

localhost> box.space[0].index[0]:alter({name="pk_v2", parts={0, 'str'}})
---
...
localhost> box.space[0].index[0]
---
- unique: true
  idx: index 0
  n: 0
  type: HASH
  key_field:
    0:
      type: NUM <-- Hasn't changed
      fieldno: 0
  name: pk_v2 <-- Has changed
  id: 0
...