Config#forget recursively removes a key from self and its default (#1899)

The following example should return 'Readline' after 'input' has been forgotten:
  # .pryrc
  Pry.config.input = RbReadline

   # Pry session
   pry(main)> _pry_.config.forget(:input)
   # This will unexpectedly return RbReadline instead of Pry's default, 'Readline'
   pry(main)> _pry_.config.input
   => RbReadline

This commit fixes the above example to return 'Readline' after the 'input' option has been forgotten, allowing it to be restored to Pry's default.

lib/pry/config/behavior.rb

@@ -187,6 +187,7 @@ class Pry
       def forget(key)
         key = key.to_s
         __remove(key)
+        default.forget(key) if default && default != last_default
       end
 
       #

spec/config_spec.rb

@@ -155,13 +155,13 @@ RSpec.describe Pry::Config do
     end
   end
 
-  describe "#forget" do
-    it "forgets a local key" do
-      local = described_class.new described_class.from_hash(foo: 1)
-      local.foo = 2
-      expect(local.foo).to eq 2
-      local.forget(:foo)
-      expect(local.foo).to eq(1)
+  describe '#forget' do
+    it 'restores a key to its default value' do
+      last_default = described_class.from_hash(a: 'c')
+      middle_default = described_class.from_hash({a: 'b'}, last_default)
+      c = described_class.from_hash({a: 'a'}, middle_default)
+      c.forget(:a)
+      expect(c.a).to eq('c')
     end
   end