Improve ruby desc snippet

Instead of dummy text, first argument for `describe` becomes a class
name derived from a file name. Example:
file:              some_class_spec.rb
describe argument: SomeClass

snippets/ruby.snippets

@@ -957,7 +957,7 @@ snippet tctss
 #     Rspec snippets     #
 ##########################
 snippet desc
-	describe ${1:class_name} do
+	describe ${1:`substitute(substitute(vim_snippets#Filename(), '_spec$', '', ''), '\(_\|^\)\(.\)', '\u\2', 'g')`} do
 		${0}
 	end
 snippet descm