Incorrect field mapping for `initialComment`

[mikeymclellan]

This is mapped as a string but is actually returned as an object from Slack and causes the deserialisation of the API response to throw a SlackException exception with the message "Array to string conversion".

https://github.com/cleentfaar/slack/blob/ba7073cb79ccea423cd68f6a69096384a730a013/src/CL/Slack/Resources/config/serializer/CL.Slack.Model.File.yml#L82

initial_comment is actually this format:

{id: "Fc80KCQ5MF",
created: 1510793564,
timestamp: 1510793564,
user: "U8167JE7P",
is_into: true,
comment: "Blah blah blah"
}