Fix invalid AST emit

* Mutant prevents double emits via guarding on source representation of
  partial AST nodes. Not all AST nodes are valid as roots, so this
  change creates a valid root around an leave.

* Over the time multiple of these situartions must be solved, we'll end
  up in a more deduplicated solution.

* Closes: #165

lib/mutant.rb

@@ -87,6 +87,7 @@ require 'mutant/mutator/node/if'
 require 'mutant/mutator/node/case'
 require 'mutant/mutator/node/splat'
 require 'mutant/mutator/node/resbody'
+require 'mutant/mutator/node/rescue'
 require 'mutant/config'
 require 'mutant/loader'
 require 'mutant/context'

lib/mutant/mutator/node/generic.rb

@@ -10,9 +10,7 @@ module Mutant
         # These nodes still need a dedicated mutator,
         # your contribution is that close!
         handle(
-          :ensure,
-          :rescue, :redo, :defined?,
-          :regopt, :retry, :arg_expr,
+          :ensure, :redo, :defined?, :regopt, :retry, :arg_expr,
           :kwrestarg, :kwoptarg, :kwarg, :undef, :module, :empty,
           :alias, :for, :xstr, :back_ref, :class,
           :sclass, :match_with_lvasgn, :match_current_line, :while_post,

lib/mutant/mutator/node/rescue.rb

@@ -0,0 +1,26 @@
+# encoding: utf-8
+
+module Mutant
+  class Mutator
+    class Node
+      # Mutator for rescue nodes
+      class Rescue < Generic
+
+        handle :rescue
+
+        # Return identity
+        #
+        # @param [Parser::AST::Node] node
+        #
+        # @return [String]
+        #
+        # @api private
+        #
+        def self.identity(node)
+          super(NodeHelpers.s(:kwbegin, node))
+        end
+
+      end # Rescue
+    end # Node
+  end # Mutator
+end # Mutant

spec/unit/mutant/mutator/node/define_spec.rb

@@ -14,6 +14,19 @@ describe Mutant::Mutator, 'def' do
     it_should_behave_like 'a mutator'
   end
 
+  context 'empty rescue body' do
+    let(:source) { "def foo\nfoo\nrescue\nend" }
+
+    let(:mutations) do
+      mutations = []
+      mutations << 'def foo; raise; end'
+      mutations << 'def foo; nil; rescue; end'
+      mutations << 'def foo; end'
+    end
+
+    it_should_behave_like 'a mutator'
+  end
+
   context 'with no arguments' do
     let(:source) { 'def foo; true; false; end' }