Deferred values don't seem to work for postgres_password (aka admin password)

Describe the Bug

When passing a Deferred value to the postgres_password parameter I get the following error message:

Failed on [...]:
  Apply failed to compile for [...]: 'postgresql::postgresql_escape' parameter 'input_string' expects a String value, got Deferred (file: /[...]/.modules/postgresql/manifests/server/passwd.pp, line: 28, column: 16)

Expected Behavior

Expected deferred values to work, that is simply setting the value to the deferred value.

Steps to Reproduce

Setup a default instance and run something similar to the below:

class { '::postgresql::server':
    postgres_password => Deferred('unwrap', [$admin_password_vault]),
}

Environment

• Version: 8.2.1 (also with 10.0.2)
• Platform: CentOS 7

Additional Context

Start at .modules/postgresql/manifests/server/passwd.pp, line 28, where the postgres_password value reaches postgresql_escape. Reproduce the default server setup with Deferred('unwrap', [$admin_password_vault]) and trace the parameter handling. Done means the deferred value is accepted and the instance applies successfully without the String-type error.