compare last modified times using 1 second resolution

Since the time formats used in the If-Modified-Since header do
only have seconds, comparing it to a file's last modification
time will fail when the filesystem has a higher resolution.

Although modern file system like ext4 and btrfs have nanosecond
timestamp resolution, the java.io.File#lastModified method just
returns timestamps with a resolution of 1 second on Linux.

See http://bugs.java.com/view_bug.do?bug_id=6939260

In contrast, on MS Windows with NTFS, the last ResourceSpec test
was indeed failing because the JVM uses a higher resolution here.
Merge pull request #220 from avdv/netty-server-mtime-seconds

compare last modified times using 1 second resolution