mj-include cannot be used directly inside mjml

[LorenzoLeonardini]

It seems like the parser raises an error when mj-include is placed directly inside mjml.

Honestly, it's an improbable edge case, the only thing I'm thinking of is someone including a common mj-head (might be used as a temporary workaround for #502), which is very unlikely as one would probably want to set mj-title and mj-preview anyway, but MJML accepts it