[ruby-core:23267] StringIO: RubySpec violation

I ran RubySpec against the 1.8.6-p368 release. It seems that 
StringIO#ungetc changed.

p368:
irb(main):001:0> io = StringIO.new("1234")
=> #<StringIO:0x208f14>
irb(main):002:0> io.ungetc(65)
=> nil
irb(main):003:0> io.string
=> "A1234"


p287:
irb(main):001:0> io = StringIO.new("1234")
=> #<StringIO:0x208fa0>
irb(main):002:0> io.ungetc(65)
=> nil
irb(main):003:0> io.string
=> "1234"

1.9:
=> #<StringIO:0x419948>
irb(main):002:0> io.ungetc(65)
=> nil
irb(main):003:0> io.string
=> "A1234"
irb(main):004:0>

Is this intentional?