Fixed a bug that made GZIPInputStream.getModifiedtime always return 0

GZIPInputStream.getModifiedtime always returns 0 because Inflate.java was writing directly to GZIPHeader.time, which is different from GZIPHeader.mtime and not actually used.  This pull request adds a test, fixes the bug, and gets rid of the unused 'time' member in GZIPHeader.

Separately, shouldn't the name of that function actually be getModifiedTime  with a capital T on Time?