UnmarshalStream: Fixed two off-by-one errors in unmarshalInt; it is now consistent with r_long in marshal.c of MRI.  This also means there are three different ways to marshal the number 0.
Merge pull request #1338 from DavidEGrayson/jruby_pull_request_1

UnmarshalStream: Fixed two off-by-one errors in unmarshalInt