restructure some specs for deflating broken zlib streams
restructure some specs for deflating broken zlib streams

I've learnt more about why we can't inflate this stream (it's a zlib
container stream with a broken adler32 checksum). Now the spec describes
the issue better.

Still no fix for it though, I can't find a way to ask the ruby zlib
bindings to ignore checksum failures.

yob/pdf-reader,yob/pdf-reader,yob/pdf-reader