Do not emit mutations if signed zero float/double

There are two representations of zero in IEEE 754. The negative and the
positive zero. This commutis removes a mutation that causes the sign to be flipped.

These mutations are very uneasy to kill. Currently only these side
effects are known:

1 / -0.0 => -Infinity
1 / 0.0 => Infinity
0.0.to_s => "0.0"
-0.0.to_s => "-0.0"

I'm happy to readd the mutation - when someone adds more wisdom to
this case ;)

Closes #126

lib/mutant/mutator/node/literal/float.rb

@@ -49,7 +49,11 @@ module Mutant
           # @api private
           #
           def values
-            [0.0, 1.0, -children.first]
+            original = children.first
+            # Work around a bug in RBX/MRI or JRUBY:
+            [0.0, 1.0, -original].delete_if do |value|
+              value.eql?(original)
+            end
           end
 
         end # Float

spec/unit/mutant/mutator/node/literal/float_spec.rb

@@ -3,25 +3,60 @@
 require 'spec_helper'
 
 describe Mutant::Mutator::Node::Literal, 'float' do
-  let(:source) { '10.0' }
-
-  let(:mutations) do
-    mutations = []
-    mutations << 'nil'
-    mutations << '0.0'
-    mutations << '1.0'
-    mutations << random_float.to_s
-    mutations << '(0.0 / 0.0)'
-    mutations << '(1.0 / 0.0)'
-    mutations << '(-1.0 / 0.0)'
-    mutations << '-10.0'
-  end
-
-  let(:random_float) { 7.123 }
 
   before do
     Mutant::Random.stub(float: random_float)
   end
 
-  it_should_behave_like 'a mutator'
+  let(:random_float) { 7.123 }
+
+  context 'positive number' do
+    let(:source) { '10.0' }
+
+    let(:mutations) do
+      mutations = []
+      mutations << 'nil'
+      mutations << '0.0'
+      mutations << '1.0'
+      mutations << random_float.to_s
+      mutations << '(0.0 / 0.0)'
+      mutations << '(1.0 / 0.0)'
+      mutations << '(-1.0 / 0.0)'
+      mutations << '-10.0'
+    end
+
+    it_should_behave_like 'a mutator'
+  end
+
+  context '0.0' do
+    let(:source) { '0.0' }
+
+    let(:mutations) do
+      mutations = []
+      mutations << 'nil'
+      mutations << '1.0'
+      mutations << random_float.to_s
+      mutations << '(0.0 / 0.0)'
+      mutations << '(1.0 / 0.0)'
+      mutations << '(-1.0 / 0.0)'
+    end
+
+    it_should_behave_like 'a mutator'
+  end
+
+  context '-0.0' do
+    let(:source) { '-0.0' }
+
+    let(:mutations) do
+      mutations = []
+      mutations << 'nil'
+      mutations << '1.0'
+      mutations << random_float.to_s
+      mutations << '(0.0 / 0.0)'
+      mutations << '(1.0 / 0.0)'
+      mutations << '(-1.0 / 0.0)'
+    end
+
+    it_should_behave_like 'a mutator'
+  end
 end