[1.9] Fixnum#** sometimes returns wrong Bignum

Fixnum#** sometimes returns wrong Bignum in 1.9 mode.

$ jruby -v --1.9 -e '10.times {|i| puts "#

{(n = 2 ** i).class} #{n}" }'
jruby 1.6.0 (ruby 1.9.2 patchlevel 136) (2011-03-15 f3b6154) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_24) [linux-amd64-java]
Fixnum 1
Fixnum 2
Fixnum 4
Bignum 8
Fixnum 16
Bignum 32
Fixnum 64
Bignum 128
Fixnum 256
Bignum 512

The following is right behavior.

$ jruby -v --1.8 -e '10.times {|i| puts "#{(n = 2 ** i).class}

#

{n}

" }'
jruby 1.6.0 (ruby 1.8.7 patchlevel 330) (2011-03-15 f3b6154) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_24) [linux-amd64-java]
Fixnum 1
Fixnum 2
Fixnum 4
Fixnum 8
Fixnum 16
Fixnum 32
Fixnum 64
Fixnum 128
Fixnum 256
Fixnum 512