(This is a manual merge of #9102)

Original implementation has bugs if the regex contains a match group.

Example:

    excerpt('This is a beautiful? morning', /\b(beau\w*)\b/i, :radius => 5)
    Expected: "...is a beautiful? mor..."
    Actual: "...is a beautifulbeaut..."

The original phrase was being converted to a regex and returning the text
either side of the phrase as expected:

    'This is a beautiful? morning'.split(/beautiful/i, 2)
    # => ["This is a ", "? morning"]

When we have a match with groups the match is returned in the array.

Quoting the ruby docs: "If pattern is a Regexp, str is divided where the
pattern matches. [...] If pattern contains groups, the respective matches will
be returned in the array as well."

    'This is a beautiful? morning'.split(/\b(beau\w*)\b/iu, 2)
    # => ["This is a ", "beautiful", "? morning"]

If we assume we want to split on the first match – this fix makes that
assumption – we can pass the already assigned `phrase` variable as the place
to split (because we already know that a match exists from line 168).

Originally spotted by Louise Crow (@crowbot) at
https://github.com/mysociety/alaveteli/pull/1557

The helper will yield each matched word, and you can use this instead of the
':highlighter' option for more complex replacing logic:

  highlight('My email is me@work.com', EMAIL_REGEXP) { |m| mail_to(m) }
  # => 'My email is <a href="mailto:me@work.com">me@work.com</a>'

fix build- see https://travis-ci.org/rails/rails/jobs/27578159#L2913.

In tag helper, honor html_safe on array parameters; also make safe_join more similar to Array.join by first calling flatten.

reviewed: @tenderlove

:precision was incorrectly being applied to Rationals

before:
  ActiveSupport::NumberHelper.number_to_rounded Rational(10, 3), precision: 2
  => "3.3"
after:
  ActiveSupport::NumberHelper.number_to_rounded Rational(10, 3), precision: 2
  => "3.33"

`options[:default]` and `options[:raise]` can be mistakenly added to the `options` hash. This can be a problem if you're reusing the same object.

rename ::_local_prefixes to ::local_prefixes to state the public attribute.
document the latter.
make ::local_prefixes private, test overriding it and remove documentation for overriding ::_parent_prefixes.

o_O

Previously, only the object and method name from the label tag were
used when looking up the translation for a label. If a value is
given for the label, this ought to be additionally used. The
following:

    # form.html.erb
    <%= form_for @post do |f| %>
      <%= f.label :type, value: "long" %>
    <% end %>

    # en.yml
    en:
      activerecord:
        attributes:
          post/long: "Long-form Post"

Used to simply return "long", but now it will return "Long-form
Post".