Add a spec for Method#source_location for core methods

Author: eregon

spec/ruby/core/method/source_location_spec.rb

@@ -86,6 +86,24 @@ def f
     method.source_location[1].should == line
   end
 
+  it "works for core methods where it returns nil or <internal:" do
+    loc = method(:__id__).source_location
+    if loc == nil
+      loc.should == nil
+    else
+      loc[0].should.start_with?('<internal:')
+      loc[1].should be_kind_of(Integer)
+    end
+
+    loc = method(:tap).source_location
+    if loc == nil
+      loc.should == nil
+    else
+      loc[0].should.start_with?('<internal:')
+      loc[1].should be_kind_of(Integer)
+    end
+  end
+
   describe "for a Method generated by respond_to_missing?" do
     it "returns nil" do
       m = MethodSpecs::Methods.new