AR::AttributeMethods#[] raises AM::AttributeMissingError for missing att...

[frodsan]

...ributes.

This fixes the following behaviour:

class Person < ActiveRecord::Base
  belongs_to :company
end

# Before:
person = Person.select('id').first
person[:name]       # => nil
person.name         # => ActiveModel::MissingAttributeError: missing_attribute: name
person[:company_id] # => nil
person.company      # => nil

# After:
person = Person.select('id').first
person[:name]       # => ActiveModel::MissingAttributeError: missing_attribute: name
person.name         # => ActiveModel::MissingAttributeError: missing_attribute: name
person[:company_id] # => ActiveModel::MissingAttributeError: missing_attribute: company_id
person.company      # => ActiveModel::MissingAttributeError: missing_attribute: company_id

Fixes #5433.

[frodsan]

/cc @rafaelfranca Sorry for the previous PR, I think I forced push without a commit lulz.