allow to Unset field of the message and composite

[alovak]

With this PR, you will be able to unset the field, mark it as 'not set,' and replace it with a new field instance to avoid any ghost values.

Here is an example of how you can use it, especially when building responses that echo most of the fields:

response, err := request.Clone()
// handle error

err = response.UnsetFields("2", "55.F9F36")
// handle error