[Form] FormBuilder::getIterator() now deals with resolved children

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I think FormBuilder::getIterator() should resolve children before make an iterator because it seems to be used in same purpose with FormBuilder::all().
What do you think?