! Keep attribute if it's already a hash

Fixes a bug with updating entries with one to many associations.

  - Scenario:
    - Collection that has many Entries and description
    - I want to update the description via `entry.update`

  - What happens:
    - `entry.fields[:entries]` array already includes the entries in the required hash format but is then parsed
    - the parser checks the type of the attribute
    - type is neither `Asset` nor `Entry`
    - `nil` replaces entry that does not require parsing

  - Suggested solution:
    - keep the entry in the array if it's already a hash