I found a bug that validation callbacks don't fire on multiple context.
So I've fixed it.
Example:
```ruby
class Dog
include ActiveModel::Validations
include ActiveModel::Validations::Callbacks
attr_accessor :history
def initialize
@history = []
end
before_validation :set_before_validation_on_a, on: :a
before_validation :set_before_validation_on_b, on: :b
after_validation :set_after_validation_on_a, on: :a
after_validation :set_after_validation_on_b, on: :b
def set_before_validation_on_a; history << "before_validation on a"; end
def set_before_validation_on_b; history << "before_validation on b"; end
def set_after_validation_on_a; history << "after_validation on a" ; end
def set_after_validation_on_b; history << "after_validation on b" ; end
end
```
Before:
```
d = Dog.new
d.valid?([:a, :b])
d.history # []
```
After:
```
d = Dog.new
d.valid?([:a, :b])
d.history # ["before_validation on a", "before_validation on b", "after_validation on a", "after_validation on b"]
```
A few have been left for aesthetic reasons, but have made a pass
and removed most of them.
Note that if the method `foo` returns an array, `foo << 1`
is a regular push, nothing to do with assignments, so
no self required.
`before_validation` and `after_validation` from
ActiveModel::Validation::Callbacks accept an optional `:on` parameter
that was not previously documented or tested. For instance given
before_validation :do_something, on: :create
then `object.valid?(:create)` will invoke `:do_something` while
`object.valid?` or `object.valid?(:anything_else)` will not.
Before this commit, returning `false` in an ActiveModel validation
callback such as `before_validation` would halt the callback chain.
After this commit, the behavior is deprecated: will still work until
the next release of Rails but will also display a deprecation warning.
The preferred way to halt a callback chain is to explicitly `throw(:abort)`.
This stems from https://github.com/rails/rails/pull/17227#discussion_r21641358
It's simply a clarification of the current behavior by which if an
`after_validation` ActiveModel callback returns +false+, then further
`after_` callbacks **are not halted**.
