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

Fixes: https://github.com/rails/rails/issues/16814
Improve performance of AR object instantiation

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.
Remove unneccesary default parameters
Removed documentation that still mentioned using respond_with in place
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.
Merge pull request #17528 from revans/respond_with_doc

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

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
Replace Html with HTML [ci skip]

- Everywhere else we using HTML Sanitizer except this place.
added example of squish!, remove, test case for multiple occurrence of
pattern removal

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

removed extra whitespaces
Merge pull request #17530 from prathamesh-sonpatki/fix-html

Replace Html with HTML [ci skip]
Merge pull request #17529 from slothbear/patch-1

Remove redundancy [ci skip]
docs, synchronize 4.2. release notes. [ci skip]

/cc @chancancode
Merge pull request #17526 from rishijain/update_docs_6

Update docs 6
Fix grouped expression warning -  `warning: (...) interpreted as grouped expression`
Merge pull request #17536 from vipulnsward/fix-group-expression-warning

Fixed grouped expression warning
Merge pull request #17523 from tgxworld/reset_session_after_calling_with_routing

Remove session to allow `with_routing` to be called twice.
tests, use SchemaDumpingHelper to dump a specific table.

This makes debugging the generated schema output much easier.
As a side effect it also shaves off 2.5 seconds of test runtime.
tests, remove unneeded requires.
added example of hash#except, and removed extra whitespaces [ci skip]
Merge pull request #17543 from rishijain/update_docs_9

added example of hash#except, and removed extra whitespaces [ci skip]
[Enh] Changed the visibility of the ActiveModel::Dirty#clear_attribute_changes method

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.