Avoid Ruby warnings "ambiguous first argument" (#881)

- add parentheses to disambiguate

spec/draper/collection_decorator_spec.rb

@@ -217,7 +217,7 @@ module Draper
         it "uses the custom class name" do
           decorator = ProductsDecorator.new([])
 
-          expect(decorator.to_s).to match /ProductsDecorator/
+          expect(decorator.to_s).to match(/ProductsDecorator/)
         end
       end
     end

spec/draper/decorator_spec.rb

@@ -439,7 +439,7 @@ module Draper
       it "returns a detailed description of the decorator" do
         decorator = ProductDecorator.new(double)
 
-        expect(decorator.inspect).to match /#<ProductDecorator:0x\h+ .+>/
+        expect(decorator.inspect).to match(/#<ProductDecorator:0x\h+ .+>/)
       end
 
       it "includes the object" do