The test was skipped because of an issue that, in the meantime,
has been fixed: https://github.com/rubinius/rubinius/issues/3328.
Using the latest Rubinius (the one currently on Travis CI), this
is the result:
```sh
$ ruby --version
rubinius 2.5.3 (2.1.0 2482b093 2015-05-10 3.5.1 JI) [x86_64-darwin14.3.0]
```
**Before this PR**
```sh
$ ruby -Itest test/cases/attribute_assignment_test.rb
Run options: --seed 58569
.....S...
Finished in 0.048278s, 186.4203 runs/s, 269.2738 assertions/s.
9 runs, 13 assertions, 0 failures, 0 errors, 1 skips
You have skipped tests. Run with --verbose for details.
```
**After this PR**
$ ruby -Itest test/cases/attribute_assignment_test.rb
Run options: --seed 35720
.........
Finished in 0.029441s, 305.6961 runs/s, 475.5273 assertions/s.
9 runs, 14 assertions, 0 failures, 0 errors, 0 skips
```
The name `ActiveModel::AttributeAssignment::UnknownAttributeError` is
too implementation specific so let's move the constant directly under
the ActiveModel namespace.
Also since this constant used to be under the ActiveRecord namespace, to
make the upgrade path easier, let's avoid raising the former constant
when we deal with this error on the Active Record side.
