Pry#binding_stack respected by :when_started hook

* If _pry_.binding_stack is set then binding_stack parameter (from :when_started) is ignored, even if it's modified.
* binding_stack parameter should eventually be removed altogther before 0.9.8 release, and people can use _pry_.binding_stack instead

lib/pry/pry_class.rb

@@ -123,8 +123,12 @@ class Pry
       pry_instance
     )
 
-    head, *tail = binding_stack
-    pry_instance.binding_stack.push(*tail)
+    if pry_instance.binding_stack.empty?
+      head, *tail = binding_stack
+      pry_instance.binding_stack.push(*tail)
+    else
+      head = pry_instance.binding_stack.pop
+    end
 
     # Enter the matrix
     pry_instance.repl(head)

test/test_hooks.rb

@@ -320,6 +320,56 @@ describe Pry::Hooks do
 
         Pry.config.hooks.delete_hook(:when_started, :test_hook)
       end
+
+      it 'should allow overriding of target (and binding_stack)' do
+        options = nil
+        o = Object.new
+        class << o; attr_accessor :value; end
+        
+        Pry.config.hooks.add_hook(:when_started, :test_hook) { |binding_stack, opt, _pry_| binding_stack.replace [Pry.binding_for(o)] }
+
+        redirect_pry_io(InputTester.new("@value = true","exit-all")) do
+          Pry.start binding, :hello => :baby
+        end
+
+        o.value.should == true
+        Pry.config.hooks.delete_hook(:when_started, :test_hook)
+      end
+
+      it 'should allow overriding of target (and binding_stack) via _pry_.binding_stack' do
+        options = nil
+        o = Object.new
+        class << o; attr_accessor :value; end
+        
+        Pry.config.hooks.add_hook(:when_started, :test_hook) { |binding_stack, opt, _pry_| _pry_.binding_stack = [Pry.binding_for(o)] }
+
+        redirect_pry_io(InputTester.new("@value = true","exit-all")) do
+          Pry.start binding, :hello => :baby
+        end
+
+        o.value.should == true
+        Pry.config.hooks.delete_hook(:when_started, :test_hook)
+      end
+
+      it 'should give precedence to _pry_.binding_stack over binding_stack' do
+        options = nil
+        o = Object.new
+        class << o; attr_accessor :value; end
+        
+        Pry.config.hooks.add_hook(:when_started, :test_hook) do |binding_stack, opt, _pry_|
+          _pry_.binding_stack = [Pry.binding_for(o)]
+          binding_stack.replace [Pry.binding_for(5)]
+        end
+
+        redirect_pry_io(InputTester.new("@value = true","exit-all")) do
+          Pry.start binding, :hello => :baby
+        end
+
+        o.value.should == true
+        Pry.config.hooks.delete_hook(:when_started, :test_hook)
+      end
+      
+      
     end
 
     describe "after_session hook" do