Deal with regex match groups in excerpt

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

Improves readability of this line...

[ci skip]

Add warning section for `rescue_from Exception` in Guides

Always assume strings with non-numeric characters change numeric types

We previously only did this if the old value was zero, to make sure
numericality validations run and failed if the user gave 'wibble' as the
value, which would be type cast to 0. However, numericality validations
will fail if there are any non-numeric characters in the string, so 5 ->
'5wibble' should also be marked as changed.

Silence warning in test

We still had one file using `column_for_attribute` when it could return
nil, causing deprecation warnings in the tests.

Need to add individual `:nodoc:` for nested classes / modules to completely
remove the constants from the API.

Add #model_name instance method to ActiveModel::Naming

Adding `# :nodoc:` to the parent `class` / `module` is not going
to ignore nested classes or modules.

There is a modifier `# :nodoc: all` but sadly the containing class
or module will continue to be in the docs.

/cc @sgrif

Add rails:update to the upgrading guides

Remove unused params

Disable autolink in example code blocks in guides. [ci skip]

/cc @chancancode @robin850

configuration reference: http://alexgorbatchev.com/SyntaxHighlighter/manual/configuration/

Related issues:

* https://github.com/vmg/redcarpet/issues/334

* https://github.com/docrails-tw/guides/issues/62

remove unnecessary require from json serialization test cases

Moved explanation of `--mountable` option up

[ci skip] correct output for asset_helper methods

Revert #15502, and add a deprecation warning instead.

s/overriden/overridden [ci skip]

This is public API, and `simple_form` depends on the `nil` return value.
We need to go through a deprecation cycle to return a null object. If
people want hash access, they can access the hash.

Change double quotes to single quotes in guide

Remove unneeded `@column_types` instance variable

/cc @sgrif

Remove old deprecation warning

This was used more previously, but other uses have been removed.

Add missing test cases for `attribute_names` instance method

`reload` should fully reload attributes

There is a class method with the same name that is tested, but not the
instance method.

`reload` is meant to put a record in the same state it would be if you
were to do `Post.find(post.id)`. This means we should fully replace the
attributes hash.

This has been around for a couple of versions now, a `NoMethodError`
should suffice at this point.

[ci skip] Added missing colon in asset manifest JSON