play -m now uses eval_string.replace() instead of changing Pry#input

This is simpler and more transparent. Also added to this commit is `show-input` now invoked immediately after a `play -m meth --open` (this was only possible because of the eval_string change).
Tests for `play` method also updated to reflect changes.

lib/pry/default_commands/input.rb

@@ -79,7 +79,8 @@ e.g amend-line puts 'hello again'   # no line number modifies immediately preced
           range = opts.l? ? one_index_range_or_number(opts[:l]) : (0..-1)
           range = (0..-2) if opts.o?
 
-          _pry_.input = StringIO.new(Array(code.each_line.to_a[range]).join)
+          eval_string.replace((code.each_line.to_a[range]).join)
+          run "show-input" if opts.o?
         elsif opts.f?
           file_name = File.expand_path(opts[:f])
           next output.puts "No such file: #{opts[:f]}" if !File.exists?(file_name)

test/test_default_commands/test_input.rb

@@ -186,17 +186,26 @@ describe "Pry::DefaultCommands::Input" do
 
     it 'should play a method with the -m switch (multiple line)' do
       $o = Object.new
-      def $o.test_method
-        1 + 102
-        5 * 6
+      class << $o
+        attr_accessor :var1, :var2
       end
 
+      def $o.test_method
+        @var1 = 20
+        @var2 = 30
+      end
+
+      obj = Object.new
+      b = Pry.binding_for(obj)
+
       redirect_pry_io(InputTester.new('play -m $o.test_method --lines 2..3', "exit-all"), str_output = StringIO.new) do
-        pry
+        b.pry
       end
 
-      str_output.string.should =~ /103\n.*30/
-      $o = nil
+      obj.instance_variable_get(:@var1).should == 20
+      obj.instance_variable_get(:@var2).should == 30
+      str_output.string.should =~ /30/
+      str_output.string.should.not =~ /20/
     end
 
   end