Fixed: test_total_colors(Image_Attributes_UT) in Image_attributes.rb fails #80

same as #108

> RMagick 1.9.1
>    o Fixed bug #2157, Image#total_colors is now an alias of Image#number_colors

<cite>[ChangeLog](https://github.com/gemhome/rmagick/blob/7b36bdc2c67b350fa6471622bd68093bb40301d3/ChangeLog#L483)</cite>