Fix channel id typo in conversation methods

TL.DR. This PR fixes this issue

Issue

Based on MessageBird's Conversations API documentation, the start_conversation method should use a parameter named ChannelId, but at the client.rb file, it's being passed a channel_id param that generates a MessageBird::ServerException when trying to use the start_conversation method

Solution

As mentioned in the referenced issue, the solution was just to change and fix the typo at the channel_id parameter: channel_id -> ChannelId, exactly the same as in the documentation.

Also fixed the tests to ensure that the change works as expected