undefined method `to_set' in 1.5.8

[ksuther]

e311225 appears to have introduced this problem.

$ ruby /Users/kent/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/xcodeproj-1.5.8/lib/xcodeproj/project/object/helpers/build_settings_array_settings_by_object_version.rb
Traceback (most recent call last):
	5: from /Users/kent/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/xcodeproj-1.5.8/lib/xcodeproj/project/object/helpers/build_settings_array_settings_by_object_version.rb:1:in `<main>'
	4: from /Users/kent/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/xcodeproj-1.5.8/lib/xcodeproj/project/object/helpers/build_settings_array_settings_by_object_version.rb:2:in `<module:Xcodeproj>'
	3: from /Users/kent/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/xcodeproj-1.5.8/lib/xcodeproj/project/object/helpers/build_settings_array_settings_by_object_version.rb:3:in `<class:Project>'
	2: from /Users/kent/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/xcodeproj-1.5.8/lib/xcodeproj/project/object/helpers/build_settings_array_settings_by_object_version.rb:4:in `<module:Object>'
	1: from /Users/kent/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/xcodeproj-1.5.8/lib/xcodeproj/project/object/helpers/build_settings_array_settings_by_object_version.rb:8:in `<class:XCBuildConfiguration>'
/Users/kent/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/xcodeproj-1.5.8/lib/xcodeproj/project/object/helpers/build_settings_array_settings_by_object_version.rb:27:in `<module:BuildSettingsArraySettingsByObjectVersion>': undefined method `to_set' for #<Array:0x00007fe45306c090> (NoMethodError)

I think to_set needs to be to_s.

[segiddins]

thanks for pointing this out, needs to be a require 'set' at the top of the file. PR incoming

[segiddins]

#573