fix date parsing when offset is negative
fix date parsing when offset is negative

dates with a positive offset can be parsed, but not those with a
negative offset.

For instance:

2017-03-02T13:21:15.733598+08:00 is parsed correctly, but
2017-03-02T13:21:15.733598-08:00 makes the parser fail.

This commit fixes this issue.

EasyPost/syslog-rfc5424-parser