Heritrix sometimes writes empty WARC records for redirects

Just noticed an oddity in our crawls. We have a WARC response with no response in it (see below). This seems to be due to the crawler getting a `HTTP 204` response.

However, I only think that because the @ikreymer's `pywb` `cdx-indexer` creates this CDX line:

```
com,facebook)/plugins/like.php?action=like&colorscheme=light&height=21&href=http://newspig.co.uk/8-reasons-to-hold-cash-markets-are-rational-until-theyre-not/&layout=button_count&show_faces=false&width=105 20180422171119 http://www.facebook.com/plugins/like.php?href=http://newspig.co.uk/8-reasons-to-hold-cash-markets-are-rational-until-theyre-not/&layout=button_count&show_faces=false&width=105&action=like&colorscheme=light&height=21 unk 204 3I42H3S6NNFQ2MSVX7XZKYAYSCX5QBYJ - - 383 10514026 BL-20180422170134461-00018-63~ukwa-h3-pulse-daily~8443.warc.gz
```

But frankly I don't understand where it's getting the `204` from!

Assuming it is really a `204` (I'll check the crawl log), the question is: What should Heritrix3 be writing to the WARC file?

```
WARC/1.0
WARC-Type: response
WARC-Target-URI: http://www.facebook.com/plugins/like.php?href=http://newspig.co.uk/8-reasons-to-hold-cash-markets-are-rational-until-theyre-not/&layout=button_count&show_faces=false&width=105&action=like&colorscheme=light&height=21
WARC-Date: 2018-04-22T17:11:19Z
WARC-IP-Address: 157.240.1.35
WARC-Payload-Digest: sha1:3I42H3S6NNFQ2MSVX7XZKYAYSCX5QBYJ
WARC-Record-ID:
Content-Length: 0

WARC/1.0
WARC-Type: request
WARC-Target-URI: http://www.facebook.com/plugins/like.php?href=http://newspig.co.uk/8-reasons-to-hold-cash-markets-are-rational-until-theyre-not/&layout=button_count&show_faces=false&width=105&action=like&colorscheme=light&height=21
WARC-Date: 2018-04-22T17:11:19Z
WARC-Concurrent-To:
WARC-Record-ID:
Content-Length: 0

WARC/1.0
WARC-Type: metadata
WARC-Target-URI: http://www.facebook.com/plugins/like.php?href=http://newspig.co.uk/8-reasons-to-hold-cash-markets-are-rational-until-theyre-not/&layout=button_count&show_faces=false&width=105&action=like&colorscheme=light&height=21
WARC-Date: 2018-04-22T17:11:19Z
WARC-Concurrent-To:
WARC-Record-ID:
Content-Type: application/warc-fields
Content-Length: 564

via: http://newspig.co.uk/8-reasons-to-hold-cash-markets-are-rational-until-theyre-not/
hopsFromSeed: LLLE
sourceTag: http://newspig.co.uk/
fetchTimeMs: 12
charsetForLinkExtraction: ISO-8859-1
outlink: https://www.facebook.com/plugins/like.php?href=http%3A%2F%2Fnewspig.co.uk%2F8-reasons-to-hold-cash-markets-are-rational-until-theyre-not%2F&layout=button_count&show_faces=false&width=105&action=like&colorscheme=light&height=21 R Location:
outlink: http://www.facebook.com/favicon.ico I =INFERRED_MISC
outlink: http://www.facebook.com/ I =INFERRED_MISC

```

Start by checking the crawl log for the reported URL and HTTP 204 status, then compare it with the sample response, request, and metadata WARC records. Determine what Heritrix3 should emit for that response and verify the resulting WARC output against that expectation.

Written by the indexing model from the issue text.