fix Rscons.set_suffix to append the given suffix if the filename has none

spec/rscons_spec.rb

@@ -24,6 +24,16 @@ describe Rscons do
     end
   end
 
+  describe ".set_suffix" do
+    it "changes the suffix to the new one" do
+      expect(Rscons.set_suffix("foo.c", ".h")).to eq("foo.h")
+    end
+
+    it "appends a suffix if the given file name does not have one" do
+      expect(Rscons.set_suffix("bazz", ".d")).to eq("bazz.d")
+    end
+  end
+
   describe ".get_system_shell" do
     before(:each) do
       Rscons.class_variable_set(:@@shell, nil)