backport History for 1.2.8 and fill in README on 1.2.6/1.2.7
[mysql] use varbinary type when appropriate in migrations (tests)

backported from :
https://github.com/rails/rails/commit/5d30e443908558713cff16cdad4b09e203c8fae8
[mysql] limit of enum columns tests

backported from:
https://github.com/rails/rails/commit/f49800071f8cd26eed8bfb5463d015fa542437e1
[mysql] :strict config option, for STRICT_ALL_TABLES on AR-4.0
[mysql] session variables support (from configuration)

backported from :
https://github.com/rails/rails/commit/97d06e8c4bd44154abda41a3f7e4bc49929a919f
[postgres] session variables support (from configuration)
+ make the adapter's API more compatible with the native Rails adapter

backported from :
https://github.com/rails/rails/commit/97d06e8c4bd44154abda41a3f7e4bc49929a919f