Add test for field overriding
Add test for field overriding

Before documenting how YAML can be abused to define common fields and
reference them elsewhere, possibly overriding some, we should test that
Data.Yaml actually behaves the way we need (that is, inversely to what
YAML 1.1 spec says: http://yaml.org/spec/1.1/#mapping/syntax).

sol/hpack,mitchellwrosen/hpack,haskell-tinc/hpack