RouteSet: simplify routes helpers generation code

Add missing requires in routes

rake < 0.9 doesn't define Rake::DSL.

Add documentation to the SpawnMethods#merge method.

Refactor GeneratedAttributes

* Move reference? method to class to remove duplicated code
* Move to_sym typecast from #initialize to .parse method (make it
  easier to refactor reference?), remove AS object/blank require
* Use []= instead of merge!({})
* Remove in? in favor of include?, remove AS object/inclusion require

Remove circular require warning

This reverts commit 7b7cc134.

Reason: docrails isn't a rails fork. See 5444ed56

[ci skip]

section

Catch nil.to_sym errors in partial_renderer, and raise ArgumentError instead

So apps that accidentally add middlewares later aren't unwittingly dumping them in a black hole.

Closes #5911

current MySQL driver fails to fetch PK name when the PK has USING option