errors_options is calculated for every *CHECKS* but it could be
calculated one time  because options and RESERVED_OPTIONS
not change during this cycle

`:within` option as alias of `:in` as documented.

Fix #7118

Since we're not directly requiring the reverse_merge extension, we can
avoid another hash creation by using default_hash.merge! instead.

There's no need to create two extra hashes with options.merge(another_hash),
with the goal of setting only one value, so lets just set it.

Also refactor validates_each to use _merge_attributes, like other
validates_* helpers do.

prevent users from unknowingly using bad regexps that can compromise security (http://homakov.blogspot.co.uk/2012/05/saferweb-injects-in-various-ruby.html)

Passing a falsey option value for a validator currently causes that validator to
be enabled, just like "true":

    ActiveModel.validates :foo, :presence => false

This is rather counterintuitive, and makes it inconvenient to wrap `validates` in
methods which may conditionally enable different validators.

As an example, one is currently forced to write:

      def has_slug(source_field, options={:unique => true})
        slugger = Proc.new { |r| r[:slug] = self.class.sluggify(r[source_field]) if r[:slug].blank? }
        before_validation slugger
        validations = { :presence => true, :slug => true }
        if options[:unique]
          validations[:uniqueness] = true
        end
        validates :slug, validations
      end

because the following reasonable-looking alternative fails to work as expected:

      def has_slug(source_field, options={:unique => true})
        slugger = Proc.new { |r| r[:slug] = self.class.sluggify(r[source_field]) if r[:slug].blank? }
        before_validation slugger
        validates :slug, :presence => true, :slug => true, :uniqueness => options[:unique]
      end

(This commit includes a test, and all activemodel and activerecord tests pass as before.)

This reverts commit fa6d921e.

Reason: Not a fan of such massive changes. We usually close such changes
if made to Rails master as a pull request. Following the same principle
here and reverting.

[ci skip]

For future reference, this is the regex I used: ^\s*#\s*\n(?!\s*#). Replace
with the first match, and voilà! Note that the regex matches a little bit too
much, so you probably want to `git add -i .` and go through every single diff
to check if it actually should be changed.

This will render the error message on :#{attribute}_confirmation instead
of on attribute itself. When rendering confirmation errors inline on the
form with form builders such as SimpleForm and Formtastic it is
confusing to the ender user to see the confirmation error message on the
attribute element. Instead it makes more sense to have this validation
error render on the confirmation field instead.

The i18n message has been updated for the confirmation validator error
message to include the original attribute name.

Orginal code by @jamescook

Closes #1352

[Rafael Mendonça França + James Cook]

Allow infinite values for validates_length_of. Particularly useful
for prettily defining an open ended range such as

validates_length_of :human_stupidity, :within => 0..Float::INFINITY