Add support for custom exception attributes

[kyrylo]

Fixes #112 (Adding to the 'params' area of airbrake.io when
Airbrake.notify not explicitly called)

[vmihailenco]

Why not to use some standard Ruby method like to_hash and/or to_json?

[kyrylo]

When somebody defines a method with this name it's already clear why. It also must correspond to our API scheme, so if this was to_hash, I would expect it to return this instead: { http_code: 404 }. But we need the params key, for example.

[vmihailenco]

I am not convinced. You use to_hash or something like that to serialize arrays, hashes and classes. You can just set notice.params[exception] = exc and let serializer do its job.

But we need the params key, for example.

Do you have any other examples?

[kyrylo]

I understand what you want to achieve, but:

1. A Hash must be returned, so we could merge it with the Notice object, so it could be filtered & truncated, if needed
2. We could achieve what you suggest only if we allowed populating the "params" bit of the payload (ignoring context, session and the rest), but how useful would that be? I think we could afford it for the sake of simpler implementation, but we still can't overcome (1)
3. How can we detect if we should append the exception to the Notice object? Currently we check if to_airbrake is defined. With your proposed API this won't be possible and we would need to do notice.params[exception] = exc for every exception, which doesn't look good. By default, to_json on exception object seems to be calling to_s, so if the user doesn't define it, then we get a string like:
   "params":{"exception":"\"MyException\""} instead of
   "params":{"exception":{"foo":"bar"}}. Because of Rails' ActiveSupport your idea would work, but for vanilla Ruby it doesn't work like that, unfortunately.

P.S. This is how the exception would look like

class MyException < StandardError do
  def initialize(params)
    @params = params
  end
end

ex = MyException.new(foo: 'bar')

# Vanilla Ruby
ex.to_json #=> "\"MyException\""

# Rails
ex.to_json #=> "{\"foo\":\"bar\"}"

[kyrylo]

I am going to merge this today unless you object.

[hale]

This is really great @kyrylo, thanks. Integrating with the branch at the moment - will let you know when I've had a chance to test end to end.

[hale]

Looking good - data defined by to_airbrake is passed into Airbrake from the custom exception as expected. Appreciate it!

[kyrylo]

Awesome, I think this change will go in, but as for the next release, I can't promise it today or tomorrow (although it's possible). So, I'd say sometime next week.

[urkle]

spelling: confuration -> configuration

[kyrylo]

Thank you, fixed!