test for 8018
do not install ruby-prof on Ruby 2.0
Update xml serialization tests to reflect a change in builder

Due to a change in builder, nil values now generates closed tags,
so instead of this:

    <pseudonyms nil=\"true\"></pseudonyms>

It generates this:

    <pseudonyms nil=\"true\"/>

Document this change in Rails so that people can track it down easily if
necessary.

Changes in Active Model, Active Record and Active Support tests.

Cherry-pick of d65adc7, 77dd3be and 146eaf3. Fix build.
Be a bit less conservative with mysql in adapter

This will allow the new mysql 2.9.0 to be used, fixing our test issues.