A lot of missing bytes when trying to write a bitmap to memory

``` ruby
memory = FreeImage::Memory.new
image = FreeImage::Bitmap.open(...)
memory.save(image, :jpeg)

memory.memory.bytes
# => "\xFF\xD8\xFF\xE0"

memory.memory.count
# => 1017134

memory.memory.size
# => 9223372036854775807
```

This returns a memory object that contains 4 bytes, but has what appears to be the correct bytes count.

If I try `image.save(memory, :jpeg)` I get the same result.
