fix endian issue for gzip.crc

[jefferis]

Quoth BDR

This invalidly assumes a little-endian platform: on Sparc (big-endian) it fails with

* checking tests ...
 Running ‘testthat.R’
ERROR
Running the tests in ‘tests/testthat.R’ failed.
Last 13 lines of output:
 Loading required package: nat.utils
 1. Failure (at test-gziputils.r#11): digest(,algo="crc32") and gzip.crc agree --
 crc1 not equal to crc2
 1 string mismatches:
 x[1]: "89f07f7b"
 y[1]: "7b7ff089"

But I am not sure why this has not failed previously. It looks like the problem is that gzip is always written little endian e. g.

• http://www.onicos.com/staff/iz/formats/gzip.html
• http://forensicswiki.org/wiki/Gzip