Improve compatibility with ActiveRecord API for singular associations

[sandersiim]

Use relevant ActiveRecord::Associations::Association methods instead of relying on public_send. This allows application code to override behaviour of model generated association methods, without changing the behaviour of the model.association().reader method, when the association is "lazy preloaded".

E.g for a model:

class Model < ActiveRecord
  has_one :link

  def link
    puts 'application code'
    super
  end
end

calling Model.lazy_preload(:link).association(:link).reader would currently print out 'application code'. This PR makes it so that it doesn't, which is the expected behaviour since that's how ActiveRecord .preload behaves.

This incompatibility initially blocked the gem usage in our codebase, but this patch solves the issue for us.

[coveralls]

Pull Request Test Coverage Report for Build 232

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-5.2%) to 94.655%

---

Totals
Change from base Build 219:	-5.2%
Covered Lines:	549
Relevant Lines:	580

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 232

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.817%

---

Totals
Change from base Build 219:	0.0%
Covered Lines:	545
Relevant Lines:	546

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 232

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.817%

---

Totals
Change from base Build 219:	0.0%
Covered Lines:	545
Relevant Lines:	546

---

💛 - Coveralls

[sandersiim]

Thanks @DmitryTsepelev!