Be smarter about handling buffer file modes

[changemewtf]

This is a fix for #119.

The test suite still runs, but I have not added a test for this fix because I'm not sure if the behavior I defined is correct.

1. Force binary mode (file.rb): I noticed that the errors went away when I opened files in binary mode, like rubyzip itself does when you pass a filename to Zip::File.open, so now it forces binary mode when you pass an IO object.
2. Abort unnecessary writes (output_stream.rb): If you don't pass a stream which was opened for writing, then this results in a "not opened for writing" error, even if you didn't actually try to write anything.

The second part is probably not correct; I imagine the best fix would be one that only bothers doing zf.write_buffer(io) if changes were actually made to the in-memory archive.

[coveralls]

Coverage decreased (-0.13%) when pulling 7c7d786 on mcantor:master into e938438 on rubyzip:master.