Uses mysql2 0.3.13 or later
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
habtm can only ever be an association reflection