I hacked around the real bug in b96813f since we were close to a release but the real problem is that -0 will change instanceconfig.defaultSeparator and https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/RubyIO.java#L706 is checking against that changed value. Since it passes the test if goes fast path which assumes a delimeter of \n.

For singlebyte values perhaps we can still go fast path but I think we should be conservative and fix this to test if it is the original default value for recordseparator and go with that (otherwise we will be doing a length check per gets.