Fix return value of setting in GC.config

gc_config_set returned rb_gc_impl_config_get, but gc_config_get also added
the implementation key to the return value. This caused the return value
of GC.config to differ depending on whether the optional hash argument is
provided or not.

gc.c

@@ -4357,7 +4357,7 @@ gc_config_set(rb_execution_context_t *ec, VALUE self, VALUE hash)
 
     rb_gc_impl_config_set(objspace, hash);
 
-    return rb_gc_impl_config_get(objspace);
+    return Qnil;
 }
 
 static VALUE

gc.rb

@@ -312,17 +312,17 @@ module GC
   #     before setting this parameter to +false+.
   #
   def self.config hash = nil
-    return Primitive.gc_config_get unless hash
-
-    if(Primitive.cexpr!("RBOOL(RB_TYPE_P(hash, T_HASH))"))
+    if Primitive.cexpr!("RBOOL(RB_TYPE_P(hash, T_HASH))")
       if hash.include?(:implementation)
         raise ArgumentError, 'Attempting to set read-only key "Implementation"'
       end
 
       Primitive.gc_config_set hash
-    else
+    elsif hash != nil
       raise ArgumentError
     end
+
+    Primitive.gc_config_get
   end
 
   # call-seq:

test/ruby/test_gc.rb

@@ -75,12 +75,9 @@ class TestGc < Test::Unit::TestCase
     GC.start
   end
 
-  def test_gc_config_setting_returns_nil_for_missing_keys
-    missing_value = GC.config(no_such_key: true)[:no_such_key]
-    assert_nil(missing_value)
-  ensure
-    GC.config(full_mark: true)
-    GC.start
+  def test_gc_config_setting_returns_config_hash
+    hash = GC.config(no_such_key: true)
+    assert_equal(GC.config, hash)
   end
 
   def test_gc_config_disable_major