rb_raise when attempting to set the GC implementation name

Instead of silently ignoring the key, we should raise to clearly tell
the user that this key is read-only.

gc.rb

@@ -306,7 +306,15 @@ module GC
   def self.config hash = nil
     return Primitive.gc_config_get unless hash
 
-    Primitive.gc_config_set 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
+      raise ArgumentError
+    end
   end
 
   # call-seq:

test/ruby/test_gc.rb

@@ -127,9 +127,7 @@ class TestGc < Test::Unit::TestCase
   def test_gc_config_implementation_is_readonly
     omit unless /darwin|linux/.match(RUBY_PLATFORM)
 
-    impl = GC.config[:implementation]
-    GC.config(implementation: "somethingelse")
-    assert_equal(impl, GC.config[:implementation])
+    assert_raise(ArgumentError) { GC.config(implementation: "somethingelse") }
   end
 
   def test_start_full_mark