Rely on nil check in Pry.rescued [Closes #31]

Post recent refactoring, it should not be possible for
enter_exception_context to be called with an invalid exception.

`rake sintax` now does this: http://showterm.io/b4b797cf4e4676223936e

lib/pry-rescue.rb

@@ -165,7 +165,6 @@ def without_bindings_below_raise(bindings)
     # @param [Array<Bindings>] bindings  The call stack
     # @return [Array<Bindings>]
     def without_duplicates(bindings)
-      bindings = Array(bindings)
       bindings.zip([nil] + bindings).reject do |b, c|
         # The eval('__method__') is there as a shortcut as loading a method
         # from a binding is very slow.

spec/pry_rescue_spec.rb

@@ -96,11 +96,10 @@
       }.should raise_error(/raiseother_exception/)
     end
 
-    it "should work if bindings is nil" do
-      PryRescue.stub(:without_bindings_below_raise).and_return nil
-      PryRescue.stub(:phantom_load_raise?).and_return false
-      PryRescue.stub(:with_program_name).and_return nil
-      PryRescue.enter_exception_context [:hi]
+    it "should output a warning if the exception was not raised" do
+      PryRescue.should_not_receive(:enter_exception_context)
+      Pry.should_receive(:warn).once
+      Pry.rescued(RuntimeError.new("foo"))
     end
   else
     it "should open at the correct point" do