Fix add into duplicated collection into binary op

* Preference is to not mutate values, even when the mutation appears to
  be "safe" as the mutated value looks "safe". Monkeypatching,
  refactoring whatever in the future can violate what appears to be "safe"
  now.

lib/mutant/cli.rb

@@ -207,7 +207,7 @@ module Mutant
     #
     # @api private
     def add(attribute, value)
-      update(attribute => config.public_send(attribute).dup << value)
+      update(attribute => config.public_send(attribute) + [value])
     end
 
     # Add matcher configuration

lib/mutant/matcher/config.rb

@@ -41,7 +41,7 @@ module Mutant
       #
       # @api private
       def add(attribute, value)
-        update(attribute => public_send(attribute).dup << value)
+        update(attribute => public_send(attribute) + [value])
       end
 
     private

lib/mutant/runner/sink.rb

@@ -79,7 +79,7 @@ module Mutant
           original = @subject_results[mutation.subject]
 
           @subject_results[mutation.subject] = original.update(
-            mutation_results: (original.mutation_results.dup << mutation_result),
+            mutation_results: (original.mutation_results + [mutation_result]),
             tests:            mutation_result.test_result.tests
           )
 

spec/unit/mutant/cli_spec.rb

@@ -221,13 +221,13 @@ Options:
       end
     end
 
-    context 'with require flag' do
-      let(:flags) { %w[--require foo] }
+    context 'with require flags' do
+      let(:flags) { %w[--require foo --require bar] }
 
       it_should_behave_like 'a cli parser'
 
       it 'configures requires' do
-        expect(subject.config.requires).to eql(%w[foo])
+        expect(subject.config.requires).to eql(%w[foo bar])
       end
     end