Fix Integer#ceildiv to respect #coerce (#7118)

Fixes [Bug #19343]

numeric.rb

@@ -278,7 +278,7 @@ class Integer
   #
   #    3.ceildiv(1.2) # => 3
   def ceildiv(other)
-    -div(-other)
+    -div(0 - other)
   end
 
   #

test/ruby/test_integer.rb

@@ -725,5 +725,9 @@ class TestInteger < Test::Unit::TestCase
 
     assert_equal(10, (10**100-11).ceildiv(10**99-1))
     assert_equal(11, (10**100-9).ceildiv(10**99-1))
+
+    o = Object.new
+    def o.coerce(other); [other, 10]; end
+    assert_equal(124, 1234.ceildiv(o))
   end
 end