function includeChainingBehavior () {
        flag(this, 'contains', true);
      }
### .include(value)

The `include` and `contain` assertions can be used as either property
based language chains or as methods to assert the inclusion of an object
in an array or a substring in a string. When used as language chains,
they toggle the `contain` flag for the `keys` assertion.

    expect([1,2,3]).to.include(2);
    expect('foobar').to.contain('foo');
    expect({ foo: 'bar', hello: 'universe' }).to.include.keys('foo');

@name include
@alias contain
@param {Object|String|Number} obj
@param {String} message _optional_
@api public