Fix on Extlib::Inflection#singular

lib/extlib/inflection.rb

@@ -282,7 +282,7 @@ def singular(word)
         regex, hash = singularization_rules
         result.sub!(regex) {|m| hash[m]}
         singular_of[word] = result
-        return result
+        return result.dup
       end
 
       # Alias for #singular (a Railism).

spec/inflection/singular_spec.rb

@@ -499,4 +499,10 @@
   it "singularizes rice => rice" do
     "rice".singular.should == "rice"
   end
+
+  it "makes dup on result" do
+    (singular = "calls".singular).should == "call"
+    singular << "should_not_be_added_to_original"
+    "calls".singular.should == "call"
+  end
 end