Fix GC.config specs to account for differences between 3.4 and 4.0

Author: eregon

core/gc/config_spec.rb

@@ -30,16 +30,30 @@
 
       it "does not change settings that aren't present in the hash" do
         previous = GC.config
-        GC.config({}).should == previous.except(:implementation)
+        GC.config({})
         GC.config.should == previous
       end
 
       it "ignores unknown keys" do
         previous = GC.config
-        GC.config(foo: "bar").should == previous.except(:implementation)
+        GC.config(foo: "bar")
         GC.config.should == previous
       end
 
+      ruby_version_is ""..."4.0" do
+        it "returns the same as GC.config but without the :implementation key" do
+          previous = GC.config
+          GC.config({}).should == previous.except(:implementation)
+        end
+      end
+
+      ruby_version_is "4.0" do
+        it "returns the same as GC.config, including the :implementation key" do
+          previous = GC.config
+          GC.config({}).should == previous
+        end
+      end
+
       it "raises an ArgumentError if options include global keys" do
         -> { GC.config(implementation: "default") }.should raise_error(ArgumentError, 'Attempting to set read-only key "Implementation"')
       end