Fix coverage for Mutant::Subject* namespace

lib/mutant/subject.rb

@@ -9,11 +9,11 @@ module Mutant
     # @return [Enumerable<Mutation>]
     # @return [undefined]
     def mutations
-      mutations = [neutral_mutation]
-      Mutator.each(node) do |mutant|
-        mutations << Mutation::Evil.new(self, wrap_node(mutant))
-      end
-      mutations
+      [neutral_mutation].concat(
+        Mutator.each(node).map do |mutant|
+          Mutation::Evil.new(self, wrap_node(mutant))
+        end
+      )
     end
     memoize :mutations
 

lib/mutant/subject/method/instance.rb

@@ -24,8 +24,7 @@ module Mutant
           # @return [self]
           def prepare
             scope.__send__(:memoized_methods).instance_variable_get(:@memory).delete(name)
-            super
-            self
+            super()
           end
 
         private

spec/unit/mutant/subject/method/instance_spec.rb

@@ -103,7 +103,7 @@ RSpec.describe Mutant::Subject::Method::Instance::Memoized do
     it_should_behave_like 'a command method'
   end
 
-  describe '#mutations' do
+  describe '#mutations', mutant_expression: 'Mutant::Subject#mutations' do
     subject { object.mutations }
 
     let(:expected) do

spec/unit/mutant/subject_spec.rb

@@ -71,7 +71,7 @@ RSpec.describe Mutant::Subject do
     subject { object.mutations }
 
     before do
-      expect(Mutant::Mutator).to receive(:each).with(node).and_yield(mutation_a).and_yield(mutation_b)
+      expect(Mutant::Mutator).to receive(:each).with(node).and_return([mutation_a, mutation_b])
     end
 
     let(:mutation_a) { double('Mutation A') }