Defaulting String properties to `nil` in LWRPs breaks in chef13

[cattywampus]

When running the test suites using Chef13, I get the following error:

Compiling Cookbooks...

================================================================================
Recipe Compile Error in /tmp/kitchen/cache/cookbooks/wlp/resources/download_feature.rb
================================================================================

Chef::Exceptions::ValidationFailed
----------------------------------
Property name must be one of: String!  You passed nil.

Cookbook Trace:
---------------
  /tmp/kitchen/cache/cookbooks/wlp/resources/download_feature.rb:39:in `class_from_file'

Relevant File Content:
----------------------
/tmp/kitchen/cache/cookbooks/wlp/resources/download_feature.rb:

 32:  #>
 33:  =end
 34:
 35:  actions :download
 36:
 37:
 38:  #<> @attribute name Specifies the name of the asset to be downloaded.
 39>> attribute :name, :kind_of => String, :default => nil
 40:
 41:  #<> @attribute directory Specifies which local directory path utilities are downloaded to when using the :download action.
 42:  attribute :directory, :kind_of => String, :default => nil
 43:
 44:  #<> @attribute accept_license Specifies whether to accept the license terms and conditions of the feature.
 45:  attribute :accept_license, :kind_of => [TrueClass, FalseClass], :default => false
 46:
 47:  default_action :download
 48:

System Info:
------------
chef_version=13.8.5
platform=ubuntu
platform_version=14.04
ruby=ruby 2.4.3p205 (2017-12-14 revision 61247) [x86_64-linux]
program_name=chef-solo worker: ppid=1540;start=05:09:16;
executable=/opt/chef/bin/chef-solo

This is because the type of the property is limited to String which is strongly enforced by Chef. Either the default needs to be removed, or the :kind_of needs to be expanded to support [String, NilClass]. Looking at the usage, removing the default seems to be appropriate in this case. This error impacts:

• install_feature resource - name property
• download_feature resource - name and directory properties.

I have a fix in place and will be happy to submit the PR, but my branch is based off of PR #50. So if that gets merged it'll be easy to roll this fix in too.