Removed Object#itself as it's implemented in ruby 2.2
Rephrase functional testing in its relation to Rails Controller Unit Testing.
[ci skip]
Merge pull request #18244 from cristianbica/remove-object-itself

Removed Object#itself as it's implemented in ruby 2.2
Document the include_all_helpers config setting.

This config setting has been available for quite a while but was not
documented.
Merge pull request #18246 from aprescott/document-include_all_helpers-setting

Document the include_all_helpers config setting
Compare http request type with test equivalent methods, and application use-cases.

Briefly explain difference between Functional and Request type tests. [ci skip]