don't convert mailer default values to procs

Invoke mailer defaults as procs only if they are procs, do not convert
with to_proc.  That an object is convertible to a proc does not mean it's
meant to be always used as a proc.  Fixes #11533
Merge pull request #11558 from kennyj/bump_mysql2_to_0_3_13

Uses mysql2 0.3.13 or later.
decouple extensions from association object state
extend by adding relationships rather than monkey patching
only flatten one side
remove unnecessary reader
AR::Base does not need to know how to create reflection objects
Stop reading the same file twice for testing different content

assert_file already gives this functionality for free by passing
multiple contents.
use the predicate method
add the missing middleware in command line guides rake about [ci skip]
habtm can only ever be an association reflection
Merge pull request #11566 from rajcybage/master

add the missing middleware in command line guides rake about [ci skip]
Updated guides with latest method [ci skip]

for blank?
extract mutation from generation method
factory methods should not have side effects.

Move model mutation to the methods that are called on the model.
Use GeneratedAttribute#column_name if #reference? scaffold_generator in _form view [Fixes #11573]
add a specific factory method rather than using new
stop relying on side effects of const_missing
Fix secure_password password_confirmation validations
Merge pull request #11107 from nettsundere/password-confirmation-fix

Fix secure_password password_confirmation validations