Add missing tests for `set_session`
Fix `set_session.to(nil)` to no longer be a no-op
FilterParamMatcher supports Regexps
Using RSpec 3 for development.
Allow delegate_method to be required independently

This allows the use of the delegate matcher without active support,
fix for issue #561.

* Change `blank?` to `to_s.empty?`
Get validates_uniqueness_of to work better with `NOT NULL` columns

Instead of having to persist the subject, like so:

  describe 'validations' do
    subject { build(:model) }
    it { validates_presence_of :required_attr }
    context "persisted" do
      # Needs to be persisted to prevent NOT NULL db error
      subject { create(:model) }
      it { validates_uniqueness_of :unique_attr }
    end
  end

You can now just write:

  describe 'validations' do
    subject { build(:model) }
    it { validates_presence_of :required_attr }
    it { validates_uniqueness_of :unique_attr }
  end

When required the matcher saves the record to the database.
Updated docs for `ValidateUniquenessOfMatcher` [ci skip]
Tweak validate_uniqueness_of docs [ci skip]
Tweak this test so it's easier to read
Tweak warning to be descriptive, not prescriptive

While we do advise against using the validates_inclusion_of or
validates_format_of validations with boolean columns, we won't judge
if you would like to do so.
validate_absence_of: cast `@attribute` to String

ActiveRecord.columns_hash returns Hash with String keys.
This is why `it { expect(offer).to validate_absence_of(:expired_at) }`
is not working.