Add abstract definition for debug_message

[garymardell]

Purpose

To make it clearer that debug_message must be implemented on Response.

Description

I am using lucky to build a microservice that accepts protobuf definitions over http so I created my own ProtobufResponse class to stream the protobuf response straight into the response io. While implementing it I discovered that you also have to implement debug_message as well as it is required by renderable:

lucky/src/lucky/renderable.cr

Lines 194 to 196 in 2c1433d

	response.debug_message.try do |message|
	Lucky::Log.debug { message }
	end

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[jwoertink]

Looks good to me! Thanks 👍