Refactor association methods + fix issue with #find_by_id

[tiagopog]

What's this PR do?

It changes the use of #find_by_id in favor of #find_by. It also refactors some portion of the code related to the associations macros (belongs_to, has_one, has_many).

TODOs

• Create an abstract method to fetch associations;
• Use #find_by rather than #find_by_* when fetching single associations;
• Pass specs.

Background context

Recently we went through a weird situation at Beauty Date while running our seeds file. It turned out that a model based on ActiveHash was not retrieving a belongs_to association for calling the underlying #find_by_id on the related model and returning nil.

I did a little debug and figured out that, unlike the #find_by_*, calling the #find_by method was actually working as expected. I don't know yet the reason for this bug to be happening apparently
only when executing the seed task, but maybe it's something related to the seed context and the metaprogamming applied while defining those dynamic finders. I sure will need to make a deeper debug on it.

For now this fork has fixed the issue and I also think that avoiding unnecessary metaprogramming is a good practice (safer, faster etc).

Manual test

# belongs_to
City.belongs_to :country
City.first.country

# has_many
Country.has_many :cities
Country.first.cities

# has_one
Country.has_one :mayor
Country.first.mayor

[serradura]

@tiagopog I'd like to recommend to invoke the primary_key method just once before of .to_sym calling. e.g:

klass = type == :belongs_to ? association_class : name.constantize
klass.primary_key.to_sym

[tiagopog]

Nice catch, @serradura 👍

[serradura]

@tiagopog similar to https://github.com/zilkey/active_hash/pull/158/files#r161891497

[tiagopog]

@zilkey uh, I was wondering here whether it's really necessary to keep the support for the old/deprecated Rails' scoped method? As you may notice in the diff I have removed this call within the has_many method.