objspace_dump: Include shareable flag

Given that the currently planned ractor local GC implementation
performance will heavilly be influenced by the number of shareable
objects it would be valuable to be able to know how many of them
are in the heap.

test/objspace/test_objspace.rb

@@ -977,6 +977,13 @@ class TestObjSpace < Test::Unit::TestCase
     assert_equal class_name, JSON.parse(json)["name"]
   end
 
+  def test_dump_include_shareable
+    omit 'Not provided by mmtk' if RUBY_DESCRIPTION.include?("+GC[mmtk]")
+
+    assert_include(ObjectSpace.dump(ENV), '"shareable":true')
+    assert_not_include(ObjectSpace.dump([]), '"shareable":true')
+  end
+
   def test_utf8_method_names
     name = "utf8_❨╯°□°❩╯︵┻━┻"
     obj = ObjectSpace.trace_object_allocations do