And added NullRelation class implementing the null object pattern for the `Relation` class.

Character classes are the specific regexp construct
to express alternation of individual characters.

Under Rails 3.1, you were allowed to pass a hash to a find_or_create
method with multiple attribute names, but this was broken as the
arguments were being improperly validated.

Remove extra attributes from HABTM join tables in AR tests

another value has been specified).

Our schema.rb is being generated with an `add_index` line similar to this:

    add_index "foo", ["foo", "bar"], :name => "xxx", :length => {"foo"=>8, "bar=>nil}

This is the same as it was on Rails 3.1.3, however, now when that
schema.rb is evaluated, its generating bad SQL in MySQL:

    Mysql::Error: You have an error in your SQL syntax; check the manual
    that corresponds to your MySQL server version for the right syntax
    to use near '))' at line 1: CREATE UNIQUE INDEX
    `xxx` ON `foo` (`foo`(8), `bar`())

This commit adds a check for nil on the length attribute to prevent the
empty parens from being output.