Fix incorrect usage of RubyString#toString()

* Due to concurrent toString() and getString() calls, this could cause
  an AssertionError.

(cherry picked from commit 829eaf4)

Author: eregon

src/main/java/org/truffleruby/core/module/ModuleFields.java

@@ -355,7 +355,7 @@ public void setAutoloadConstant(RubyContext context, Node currentNode, String na
                     filename));
         }
         final ReentrantLockFreeingMap<String> fileLocks = getContext().getFeatureLoader().getFileLocks();
-        final ReentrantLock lock = fileLocks.get(filename.toString());
+        final ReentrantLock lock = fileLocks.get(filename.getJavaString());
         if (lock.isLocked()) {
             // We need to handle the new autoload constant immediately
             // if Object.autoload(name, filename) is executed from filename.rb