merges r24091 from trunk into ruby_1_9_1.

--

* lib/prime.rb (Prime#prime_division): now decomposes
  negative integer into a decomposition with element [-1, 1].

* test/test_prime.rb: test for it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_1@24121 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/prime.rb

@@ -194,7 +194,12 @@ class Prime
   #
   def prime_division(value, generator= Prime::Generator23.new)
     raise ZeroDivisionError if value == 0
-    pv = []
+    if value < 0
+      value = -value
+      pv = [[-1, 1]]
+    else
+      pv = []
+    end
     for prime in generator
       count = 0
       while (value1, mod = value.divmod(prime)