add CommandSet#[]= spec.

lib/pry/command_set.rb

@@ -339,7 +339,7 @@ class Pry
       if command.equal?(nil)
         return commands.delete(pattern)
       end
-      unless command < Pry::Command
+      unless Class === command && command < Pry::Command
         raise TypeError, "command is not a subclass of Pry::Command"
       end
       bind_command_to_pattern = pattern != command.match

spec/command_set_spec.rb

@@ -30,6 +30,10 @@ describe Pry::CommandSet do
       @set["help-1"] = @set["help"]
       @set["help-1"].match.should == "help-1"
     end
+
+    it "raises a TypeError when command is not a subclass of Pry::Command" do
+      lambda { @set["help"] = "hello" }.should.raise TypeError
+    end
   end
 
   it 'should call the block used for the command when it is called' do