Add spec for Compress::Gzip::Writer with extra

[straight-shoota]

This adds a spec to ensure that writing an extra header field works.

Follow-up to #14550

Note the generated gzip data is not binary identical to the test.gz fixture, so we cannot directly compare the written gzip data.
Instead, the spec reads the data and verifies it has the same content as test.gz.
I believe the difference is because Compress::Gzip::Writer does not include a CRC for the header data (ref C implementation).

Compress::Gzip::Writer:
00000000  1f 8b 08 1c 4d 7b 46 50  00 03 05 00 01 02 03 04  ....M{FP........
00000010  05 74 65 73 74 2e 74 78  74 00 68 61 70 70 79 20  .test.txt.happy
00000020  62 69 72 74 68 64 61 79  00 f3 cf 4b e5 0a 29 cf  birthday...K..).
00000030  07 00 09 b9 78 f5 07 00  00 00                    ....x.....

test.gz:
00000000  1f 8b 08 1f 4d 7b 46 50  02 03 05 00 01 02 03 04  ....M{FP........
00000010  05 74 65 73 74 2e 74 78  74 00 68 61 70 70 79 20  .test.txt.happy
00000020  62 69 72 74 68 64 61 79  00 a1 2d f3 cf 4b e5 0a  birthday..-..K..
00000030  29 cf 07 00 09 b9 78 f5  07 00 00 00              ).....x.....