Add Event#with and Event#to_h methods

[orien]

Context

As part of the feature to add causation_id to events emitted from reactors I found it's not so easy. We don't want to mutate events, instead we want an easy way to duplicate events while making the specified changes.

Changes

• Added Event#to_h method. Pretty straight-forward: returns a hash of the event attributes.

• Added Event#with method. This provides a way to create a new event identical to the old event except for the provided changes. Example:

  old_event = EventSourcery::Event.new(type: "item_added", causation_id: nil)
  new_event = old_event.with(causation_id: "05781bd6-796a-4a58-8573-b109f683fd99")
  
  new_event.type # => "item_added"
  new_event.causation_id # => "05781bd6-796a-4a58-8573-b109f683fd99"
  
  old_event.type # => "item_added"
  old_event.causation_id # => nil

  Of course, with can accept any number of event attributes:

  new_event = old_event.with(id: 42, version: 77, body: { 'attr' => 'value' })