Add more tests, ensure we never yield outside the fiber context and that we swap buffers when moving from parent to child.

Yo dawg, I heard you like streaming. So I put a fiber, inside a block, inside a body, so you can stream.

Return nil from read_attribute(:foo) if 'foo' is not present in the @attributes hash, but the _foo method has been defined. This brings the behaviour into line with the 3-0-stable branch and the master branch before 93641ed6 (there were previously no assertions about this which is why the change slipped through). Note that actually calling the 'foo' method will still raise an error if the attribute is not present.

Undo performances regressions I introduced in bbe0a507 and add test for an edge case. Add comments to explain the intent of the code.

Remove unnecessary code from define_read_method and add assertion to make sure the underscored version is actually generated

bcrypt will encrypt anything, so validate_presence_of would not catch nil / blank passwords.  Thank you to Aleksander Kamil Modzelewski for reporting this

Extract the constraint-building for joins in JoinAssociation into a separate method to make it easy to change/override (requested by Ernie Miller so that MetaWhere can add to it easily)