Add unit tests for DEFAULT_CONTROL_D_HANDLER

DEFAULT_CONTROL_D_HANDLER handles a user's Control-D (^D) presses. Add
unit tests for ^D presses. Note that this commit doesn't include unit
tests for the situation when ^D is being pressed in an expression[1].

[1]:
Example of the untested situation:
  % pry
  [1] pry(main)> class Foo
  [1] pry(main)*   ^D
  [2] pry(main)>

Signed-off-by: Kyrylo Silin <kyrylosilin@gmail.com>

lib/pry.rb

@@ -106,7 +106,7 @@ class Pry
       # clear input buffer
       eval_string.replace("")
     elsif _pry_.binding_stack.one?
-      # ^D at top-level breaks out of loop
+      # ^D at top-level breaks out of a REPL loop
       _pry_.binding_stack.clear
       throw(:breakout)
     else

test/test_control_d_handler.rb

@@ -0,0 +1,45 @@
+require 'helper'
+
+describe Pry::DEFAULT_CONTROL_D_HANDLER do
+  describe 'control-d press' do
+    before do
+      @control_d = "Pry::DEFAULT_CONTROL_D_HANDLER.call('', _pry_)"
+      @binding_stack = "self.binding_stack = _pry_.binding_stack.dup"
+    end
+
+   describe 'in an expression' do
+      it 'should clear out passed string' do
+        str = "hello world"
+        Pry::DEFAULT_CONTROL_D_HANDLER.call(str, nil)
+        str.should == ""
+      end
+    end
+
+    describe 'at top-level session' do
+      it 'should break out of a REPL loop' do
+        instance = nil
+        redirect_pry_io(InputTester.new(@control_d)) do
+          instance = Pry.new
+          instance.repl
+        end
+
+        instance.binding_stack.should.be.empty
+      end
+    end
+
+    describe 'in a nested session' do
+      it 'should pop last binding from the binding stack' do
+        base = OpenStruct.new
+        base.obj = OpenStruct.new
+
+        redirect_pry_io(InputTester.new("cd obj", "self.stack_size = _pry_.binding_stack.size",
+                                        @control_d, "self.stack_size = _pry_.binding_stack.size", "exit-all")) do
+          Pry.start(base)
+        end
+
+        base.obj.stack_size.should == 2
+        base.stack_size.should == 1
+      end
+    end
+  end
+end