Fix tracking subclasses with additional fields

[cgriego]

If you have subclasses with their own fields, none of those fields are being tracked because the options will only look at the parent class' fields. For example:

class Person
  include Mongoid::Document
  include Mongoid::History::Trackable

  track_history
  field :name, type: String
end

class Employee < Person
  field :job_title, type: String # this would never be tracked
end

This worked prior to this commit: 774dd0d

[coveralls]

Coverage increased (+0.004%) to 99.378% when pulling c00e6c7 on getaroom:fix-subclass-tracking into 3f98e77 on mongoid:master.

[dblock]

Thank you! Thoughts on below?

[dblock]

This looks strange. Change it to:

return if @prepared 

@prepared = ...

Generally I am a bit concerned that we are now making assignment of prepared and modifying it all over the place. Is there a way to fix the issue without doing that change and keep gathering settings and returning modified version from functions, then calling merge on the results?

[cgriego]

I really feel like the library took a turn down a wrong path with the linked commit and I'm just trying to make the best of it. @options was assigned and modified all over the place already, I just switched it so the original options could be retrieved. Another direction I considered was having Options keep and expose the original_options and leave it modifying @options and using @prepared in the same way it does now.

[dblock]

I like keeping original_options, feels cleaner, no?

[dblock]

Also I am fine with the code as is. LMK when you want to merge (on green).

[cgriego]

This is an uglier changeset in the commit, but it feels a little cleaner to me to in the end to keep the mutation done by prepared isolated to @prepared instead of @options.

The Travis build is green, but neither time has it updated the GitHub status. 😕

[dblock]

LGTM, merged