Stringify variables names for mysql connections

For mysql2/mysql adapters, `sql_mode` variable name set in `database.yml`
as string, was ignored and `sql_mode` was set to use strict mode.

Fixes #14895
[skip ci] Document: required `via` option in `match` routing method.
Merge pull request #14915 from juanpastas/patch-1

Update mapper.rb
[ci skip] Security guide clarity.
Merge pull request #14931 from heironimus/security_guide_clarity

[ci skip] Security guide clarity.
always use File.join
Merge pull request #14932 from phoet/use_file_join

always use File.join in asset_path
Merge pull request #14896 from jetthoughts/14895_overide_strict_by_sql_mode

Symbolize variables of mysql/mysql2 connection configuration

Conflicts:
	activerecord/CHANGELOG.md
never merge url options in to the first data hash

if you want options, don't mix them with the first hash, just pass them
all in with the second hash
always pass options to the _url method
only add the optiosn if they are not empty

I think this is wrong, but it gets the build passing for now. We should
always add options, but we need to make more guarantees about how the
underlying url helper is called
Form_tag_helper grammar fix [skip ci]
Merge pull request #14939 from jonatack/patch-8

Form_tag_helper grammar fix [skip ci]
Enhance a bit a few changelog entries [ci skip]
Follow up to 38ad5438 [ci skip]

This commit adds new lines between titles, paragraphs and code blocks
for an easier reading.

Moreover, in order to be consistent with the other guides, let's remove
the sharps on the right side of the different titles.

The "Running a Single Test" part was also explained after the "Testing
Active Record" one which shows how to run a single test under the scope
of a specific adapter.

Finally, let's explain the `-n` option to run a specific test method.