Consider an autoload resolved if #resolve didn't return nil in Autolo…

…ad#call

This fixes cases of autoload "errors" mentioned by @robin850 in
#2934 (comment)
and https://gist.github.com/robin850/3eeaed2538f50a9887c2

This is an explanation of the issue:
We already have A::B been autoloaded by the main thread. When c.rb (see
any of the previous links) is being autoloaded in another thread, class
A::B is being opened using Rubinius.open_class_under (through
Rubinius.open_class).
Since module A has an Autoload entry for B in its constant_table,
open_class_under tries to call #call on this Autload object, which returns nil
because #resolve returns false (CodeLoader.require returns false
if a feature is already been loaded).
With nil returned, open_class_under decided to create a new Class object
for B, which means the autoload entry it already had for :C is lost,
resulting in constant A::B::C not being found.

core/autoload.rb

@@ -40,7 +40,7 @@ def call(under, honor_require=false)
     if !undefined.equal?(constant) && Thread.current == thread
       constant
     else
-      worked = resolve
+      worked = !resolve.nil?
 
       if !honor_require or worked
         find_const under