Dash used instead of underscore in variable name

`content_type` variable spelled as `content-type` [here](https://github.com/puppetlabs/ruby-hocon/blob/1.2.5/lib/hocon/impl/parseable.rb#L235) and [here](https://github.com/puppetlabs/ruby-hocon/blob/1.2.5/lib/hocon/impl/parseable.rb#L260)