[ci skip] modify default host in command line guide

Print out a meaningful error when ActiveRecord::ReadOnlyRecord is raised

Currently, there is no messages which get printed out. Convoluted system
may have hooks that create other objects in which case we only fail with
no messages. This commit changes this information allowing you to know
which object is the one that actually raised the error.

[Enh] Made the ActiveModel::Dirty#clear_attribute_changes method public

In Rails 4.2 it is impossible to define a custom default value for a model's
attribute without making it appear as _changed?, especially when the model
is first initialized. Making this method publicly visible will allow such a behaviour,
without the need to use private APIs.

added example of hash#except, and removed extra whitespaces [ci skip]

This makes debugging the generated schema output much easier.
As a side effect it also shaves off 2.5 seconds of test runtime.

Remove session to allow `with_routing` to be called twice.

Fixed grouped expression warning

Update docs 6

/cc @chancancode

Remove redundancy [ci skip]

Replace Html with HTML [ci skip]

pattern removal

added example for string#remove and test case for remove of multiple occurence of pattern

removed extra whitespaces

- Everywhere else we using HTML Sanitizer except this place.

I love this guide! 
and "visually see" is redundant.
remove "visually", then the sentence reads:
> Action Mailer previews provide a way to see how emails look

[docs] Removed respond_with documentation as it was deprecated and removed from Rails [ci-skip]

of respond_to. respond_with was moved into the responders gem and deprecated
inside rails, so there is no need to mention it within rails itself.

We introduced a performance hit by adding an additional iteration
through a model's attributes on creation. We don't actually need the
values from `Result` to be a hash, we can separate the columns and
values and zip them up ourself during the iteration that we have to do.

Fixes: https://github.com/rails/rails/issues/16814

Cleanup loaded features and constants after dependency tests.

copy reflection_scopes’s unscoped value when building scope for preloading

Add test/jobs files in `rake stats`

As we are no more displaying frameworks

[ci skip]

Remove skip-gems from 4-2 release notes [ci skip]

rake db:create shows underlying error message.

Remove unused duplicated method `add_column_position` from AbstractMysqlAdapter.

- This option was removed in https://github.com/rails/rails/commit/bf17c8a531bc80.
- [ci skip]