Fix functions as default values in records

successful check_value/2 needs to return its second argument

before:

iex(1)> defrecord Baz, foo: &Dict.get/1, bar: 4711
iex(2)> IO.puts inspect Baz[]
Baz[foo: nil, bar: 4711]
:ok
iex(3)> 

after:

iex(4)> defrecord Baz, foo: &Dict.get/1, bar: 4711                 
iex(5)> IO.puts inspect Baz[]                                      
Baz[foo: &Dict.get/1, bar: 4711]
:ok
iex(6)>