Fiber#transfer previously made it impossible to resume the fiber
if it was transferred to (no resuming the target of Fiber#transfer).
However, the documentation specifies that you cannot resume a fiber
that has transferred to another fiber (no resuming the source of
Fiber#transfer), unless control is transferred back.
Fix the code by setting the transferred flag on the current/source
fiber, and unsetting the transferred flag on the target fiber.
Fixes [Bug #9664]
Fixes [Bug #12555]
* Other ruby implementations use the spec/ruby directory.
[Misc #13792] [ruby-core:82287]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59979 b2dd03c8-39d4-4d8f-98ff-823fe69b080e