Enforce UTF-8 encoding

[louiscb]

Is it possible to enforce UTF-8 encoding on the input? E.g for Jason when passing in a non UTF-8 character such as <<202>> we get an error whilst Poison gives us useless output:

iex(15)> Poison.encode!(%{foo: <<202>>})
<<123, 34, 97, 100, 109, 34, 58, 34, 202, 34, 125>>
iex(16)> Jason.encode!(%{foo: <<202>>})
** (Jason.EncodeError) invalid byte 0xCA in <<202>>
    (jason 1.5.0-alpha.2) lib/jason.ex:164: Jason.encode!/2
    iex:16: (file)