Avoid use of self.class.new(self) in Set#collect!

That prevents infinite recursion when a subclass of Set uses
`collect!` in its constructor.

This should fix [Bug #12437].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60317 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/set.rb

@@ -378,7 +378,9 @@ class Set
   # Returns an enumerator if no block is given.
   def collect!
     block_given? or return enum_for(__method__) { size }
-    replace(self.class.new(self) { |o| yield(o) })
+    set = self.class.new
+    each { |o| set << yield(o) }
+    replace(set)
   end
   alias map! collect!