archive/tar: fix parsePAX to be POSIX.1-2001 compliant
Relevant PAX specification:
<<<
If the <value> field is zero length, it shall delete any header
block field, previously entered extended header value, or
global extended header value of the same name.
>>>
We don't delete global extender headers since the Reader doesn't
even support global headers (which the specification admits was
a controversial feature).