Fix a few typos and made some tiny improvements. Refs #16576.

[Zachary Scott & Robin Dupret]

- don't mutate PATH_INFO in env, test
- test fallback content type matches Rack::File
- change assertion style
- make HTTP_ACCEPT_ENCODING comparison case insensitive
- return gzip path from method instead of true/false so we don't have to assume later
- don't allocate un-needed hash.

Original comments:

https://github.com/rails/rails/commit/
cfaaacd9#commitcomment-7468728

cc @jeremy

I put a more up-to-date description of Web Console in the release notes.
I'm not really happy with the language, so if you guys can help me with
that I'll be grateful :)

Remove duplicate line in Railties.

[ci skip]

This reverts commit 699babe8.

Also change the upgrading_ruby_on_rails link back to original in 4_2_release_notes.

[Godfrey Chan, Genadi Samokovarov]

Previously setting simple values to the config.x object resulted in the
following:

    config.x.super_debugger = true
    config.x.super_debugger #=> {}

Which was against the examples showed in the changelog/release notes.

Godfrey Chan, Genadi Samokovarov

This reverts commit de489134.

Conflicts:
	railties/lib/rails/railtie/configuration.rb

It added regression. Will be back after the beta

Mention the web-console inclusion in the default Gemfile in the Release
notes and the railties changelog. We can eventually mention it in the
upgrade guide, if needed.

[ci skip]

1. Hashes can be assigned
2. We don't need a special level anymore

The method chain only works in the top level.

If users need a second level they need to assign a OrderedOptions to the
key:

    config.resque.server = ActiveSupport::OrderedOptions.new
    config.resque.server.url = "http://localhost"
    config.resque.server.port = 3000

[Rafael Mendonça França + Carlos Antonio da Silva]

in the 4.2 Release Notes [skip ci]

Change to link suggested by @zzak [skip ci]

The error was moved outside of the `ClassMethods` module.

*   Leave two blank lines between sections/packages for readibility.

*   Always indent bullet points with four spaces. This allows multi-paragraph
    points to align properly (see #16103), and it is the same as the format we
    already use in CHANGELOGs. Doing this consistently sets an easy precedent
    for future contributors to follow.

*   Always put the PR/commit link on its own line in the source. If the bullet
    point spans multiple paragraphs, put it on its own paragraph at the end.

[Godfrey Chan & Juanito Fatas]

Also reordered some of the items to put newer ones on top (same order as
CHANGELOGs), which makes it easier to diff while we are still working on it.

This way when we do next release notes, this would be easier to copy and change.