Allow Payload to be case-insensitive

[jwoertink]

Crystal's JSON is case-sensitive, which means this throws an error:

struct Payload
  include JSON::Serializable
 
  @[JSON::Field]
  getter command : String
end

# Unhandled exception: Missing JSON attribute: command
Payload.from_json(%({"Command": "Subscribe"}))

Also, since we're checking against a case sensitive value, that Subscribe also fails

cable/src/cable/connection.cr

Line 126 in bfdb55d

return subscribe(payload) if payload.command == "subscribe"