setup Bundler in engines `bin/rails` stub.

This is necessary when bundling gems locally using `BUNDLE_PATH`.
Without this patch `bin/rails` fails with:

```
/Users/senny/.rbenv/versions/2.0.0-p353/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- rails/all (LoadError)
	from /Users/senny/.rbenv/versions/2.0.0-p353/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from bin/rails:7:in `<main>'
```
set encoding to binmode for pipe
Update Changelog, Spring is under rails/spring [ci skip]
Merge pull request #13762 from robertomiranda/spring-under-rails

Update Changelog, Spring is under rails/spring [ci skip]
Merge pull request #13745 from nextgearcapital/master

set encoding for pipe to binary mode
Add link to upgrade guide for full list of deprecated finders [ci skip]
Update Error Message when secrets.secret_key_base isn't given
Merge pull request #13765 from robertomiranda/secrets_token

Update Error Message when secrets.secret_key_base isn't given
Correcting link that is pointing to Rails blog [ci skip]
Merge pull request #13768 from rajeshthummalapally/rails_blog_correct_patch_link

Fix for incorrect link that is pointing to Rails blog [ci skip]
Fix a typo that says you application [ci skip]
Merge pull request #13769 from arelenglish/fix_typo_upgrad_guide

Fix a typo that says you application [ci skip]
Merge pull request #13767 from zzak/doc_4_1_release_notes_deprecated_finders

Add link to upgrade guide for full list of deprecated finders [ci skip]
Fix secrets.yml path in exception message

The file is config/secrets.yml, not config/initializers/secrets.yml.
let's link to the gem README not to the upgrading guide. [ci skip]

The PR #13767 added link to the upgrading guide to explain
details about activerecord_deprecated-finders. However the link target
features a stack of changes not releated at all. Also the relevant details
are not very informative.

I think we better link to the README so people can see what the gem is about
and how to use it.

/cc @chancancode @zzak
docs, hyperlink the `Mail gem` in Action Mailer guide. [ci skip]
docs should say `email` not `Email`. [ci skip]
Automatically convert dashes to underscores for url helpers
Automatically convert dashes to underscores in shorthand routes
Tidy up tests and CHANGELOG for #12598