Fix metadata backwards compat to handle deep nesting.

This is a bit confusing, but the behavior in 2.x was:

* a group's `metadata` did not have it's computed
  keys directly available; they were exposed off of
  the `:example_group` subhash.
* The `:example_group` subhash had a `:example_group`
  key that exposed the parent group's metadata
  (both user metadata and computed keys).

Before this, we were inserting extra `:example_group`
subhashes between the layers.

Author: myronmarston

lib/rspec/core/metadata.rb

@@ -370,7 +370,7 @@ class LegacyExampleGroupHash
 
       def initialize(metadata)
         @metadata = metadata
-        self[:example_group] = metadata[:parent_example_group]
+        self[:example_group] = metadata[:parent_example_group][:example_group]
       end
 
       def to_h

spec/rspec/core/metadata_spec.rb

@@ -460,13 +460,29 @@ def value_for(*args)
               describe("nested") { child = metadata }
             end
 
-            expect(child[:example_group][:example_group]).to include(
+            expect(child[:example_group][:example_group].to_h).to include(
               :foo => 3,
               :description => "Object group",
               :line_number => parent_line
             )
           end
 
+          it "works properly with deep nesting" do
+            inner_metadata = nil
+
+            RSpec.describe "Level 1" do
+              describe "Level 2" do
+                describe "Level 3" do
+                  inner_metadata = metadata
+                end
+              end
+            end
+
+            expect(inner_metadata[:example_group][:description]).to eq("Level 3")
+            expect(inner_metadata[:example_group][:example_group][:description]).to eq("Level 2")
+            expect(inner_metadata[:example_group][:example_group][:example_group][:description]).to eq("Level 1")
+          end
+
           it 'can mutate attributes when accessing them via [:example_group]' do
             meta = nil