Allow precision option for postgresql datetimes

This patch addresses the difficulty of retrieving datetime fields. By default, the database holds a higher precision than the time as a String.

This issue is discussed at length at the following links:
- [#3519](https://github.com/rails/rails/issues/3519)
- [#3520](https://github.com/rails/rails/issues/3520)

Also, kudos to @mattscilipoti

change param name to improve documentation

The keys of the error messages are actually attribute names. It makes
the documentation easier to understand:

    # Returns +true+ if the error messages include an error for the given
    # +attribute+, +false+ otherwise.
    #
    #   person.errors.messages # => { :name => ["can not be nil"] }
    #   person.errors.include?(:name) # => true
    #   person.errors.include?(:age)  # => false
    def include?(attribute)
      (v = messages[attribute]) && v.any?
    end

Dynamic finders for aliased attributes

previously dynamic finders only worked in combination with the actual
column name and not its alias defined with #alias_attribute

Allow ActiveRecord::Relation#pluck to accept multiple columns. Returns an
array of arrays containing the type casted values:

    Person.pluck(:id, :name)
    # SELECT people.id, people.name FROM people
    # [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']]

Closes #6500

Ensure it works with mix of symbols and strings, and with a select
clause possibly containing more than one column.

Also remove support for pluck with an array of columns, in favor of
passing the list of attributes:

    Model.pluck(:a, :b)

See comments: https://github.com/rails/rails/pull/6500#issuecomment-6030292

When generating xml with a custom implementation of serializable_hash,
if using the :except option, it was being overriden by the default AR
implementation that attempts to ignore the inheritance column from STI
automatically. So, if you have an implementation like this:

    def serializable_hash(options={})
      super({ except: %w(some_attr) }.merge!(options))
    end

The :except option was correctly being used for :json generation, but
not for :xml, because the options hash already contained the :except
key with the inheritance column, thus overriding the customization.

This commit fixes this problem by removing the :except logic from the
xml serializer, that happened before calling serializable_hash. Since
serializable_hash also does the same check for inheritance column, this
logic was duplicated in both places, thus it's safe to remove it from
xml serializer (see ActiveRecord::Serialization#serializable_hash).

This is an attempt to solve issue #2498, that claims for a
"Single transformation API for both xml and json representations".

ActiveRecord json/xml serialization should use as base
serializable_hash, provided by ActiveModel. Add some more coverage
around options :only and :except for both json and xml serialization.

Improve the derivation of HABTM join table name to take account of nesting.
It now takes the table names of the two models, sorts them lexically and
then joins them, stripping any common prefix from the second table name.

Some examples:

  Top level models
  (Category <=> Product)
  Old: categories_products
  New: categories_products

  Top level models with a global table_name_prefix
  (Category <=> Product)
  Old: site_categories_products
  New: site_categories_products

  Nested models in a module without a table_name_prefix method
  (Admin::Category <=> Admin::Product)
  Old: categories_products
  New: categories_products

  Nested models in a module with a table_name_prefix method
  (Admin::Category <=> Admin::Product)
  Old: categories_products
  New: admin_categories_products

  Nested models in a parent model
  (Catalog::Category <=> Catalog::Product)
  Old: categories_products
  New: catalog_categories_products

  Nested models in different parent models
  (Catalog::Category <=> Content::Page)
  Old: categories_pages
  New: catalog_categories_content_pages

Also as part of this commit the validity checks for HABTM assocations have
been moved to ActiveRecord::Reflection One side effect of this is to move when
the exceptions are raised from the point of declaration to when the association
is built. This is consistant with other association validity checks.

add :nodoc: to AM::AttributeMethods#instance_method_already_implemented?...

Make the Resolver template cache threadsafe

add :nodoc: to AM::AttributeMethods::AttributeMethodMatcher

changed the firm of changes_from_zero_to_string?

Inflector: Added missing tests cases to ensure idempotency of singularize and pluralize

Removed warnings.

1. Change in test name as already defined.
2. ambiguous first argument; put parentheses or 
even spaces

This is a follow up to #4719. It appears that singularize and pluralize
are supposed to be idempotent - i.e. when you call singularize or
pluralize multiple times on the same string, you should get the same
result. (At least for the "officially supported" cases that the stock
inflector is designed to handle.) #4719 added the missing tests for
regular cases, and this commit added the missing tests for the
irregularities.

While I'm at that, I also synced up the irregularity test cases with
the current set of irregularity cases that we ship out-of-the-box.