omitted desc + default_command no longer crashes

Seems if you omit the description for a comamnd, and then make
that command the default command in a subcommand setup,
we tried to plus nil, which doesn't work.

Hopefully closes #131

lib/gli/commands/help_modules/command_help_format.rb

@@ -103,7 +103,7 @@ def global_flags_and_switches
         def format_subcommands(command)
           commands_array = @sorter.call(command.commands_declaration_order).map { |cmd| 
             if command.get_default_command == cmd.name
-              [cmd.names,cmd.description + " (default)"] 
+              [cmd.names,String(cmd.description) + " (default)"] 
             else
               [cmd.names,cmd.description] 
             end

test/tc_help.rb

@@ -151,6 +151,34 @@ class TestApp
     }
   end
 
+  test_that "omitting default_description doesn't blow up" do
+    Given {
+      app = TestApp.new
+      app.instance_eval do
+        command :top do |top|
+          top.command :list do |list|
+            list.action do |g,o,a|
+            end
+          end
+
+          top.command :new do |new|
+            new.action do |g,o,a|
+            end
+          end
+
+          top.default_command :list
+        end
+      end
+      @command = GLI::Commands::Help.new(app,@output,@error)
+    }
+    When {
+      @code = lambda { @command.execute({},{},['top']) }
+    }
+    Then {
+      assert_nothing_raised(&@code)
+    }
+  end
+
 private
 
   def a_GLI_app(omit_options=false)