bug#23421: parse-rfc-822-date is too strict

[Ricardo Wurmus]

The procedure “parse-rfc-822-date” in the module (web http) is a bit too
strict as it rejects dates in which the hour field is a single digit
without zero-padding.

A date like this will be rejected:

    Sun, 06 Nov 1994  8:49:37 GMT

Whereas a date like this is accepted:

    Sun, 06 Nov 1994 08:49:37 GMT

The only difference is the leading zero in the timestamp.

Dates like the former are produced by maven (see “Last-Modified”
header):

~~~~~~~~~~~~~~~~~~~~
rwurmus in guix: wget -S  
http://central.maven.org/maven2/org/osgi/org.osgi.core/6.0.0/org.osgi.core-6.0.0-sources.jar
--2016-05-02 11:21:06--  
http://central.maven.org/maven2/org/osgi/org.osgi.core/6.0.0/org.osgi.core-6.0.0-sources.jar
Resolving central.maven.org (central.maven.org)... 185.31.17.209
Connecting to central.maven.org (central.maven.org)|185.31.17.209|:80... 
connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 200 OK
  x-amz-meta-last-modified-epoch: 1406692062000
  ETag: "470145ab68a6738780bd86e1c4b53d4b"
  Content-Type: application/java-archive
  Last-Modified: Wed, 30 Jul 2014  3:47:42 GMT
  X-Checksum-MD5: 470145ab68a6738780bd86e1c4b53d4b
  X-Checksum-SHA1: 347531f45363ed10e222b03231d49bb7df016a4d
  Via: 1.1 varnish
  Fastly-Debug-Digest: 
843a9842225e10fdcdc029bcd46bfe2a74c8b74a8a748e971d9779296cd6405f
  Content-Length: 296717
  Accept-Ranges: bytes
  Date: Mon, 02 May 2016 09:21:06 GMT
  Via: 1.1 varnish
  Age: 185269
  Connection: keep-alive
  X-Served-By: cache-iad2131-IAD, cache-fra1242-FRA
  X-Cache: HIT, HIT
  X-Cache-Hits: 1, 1
  X-Timer: S1462180866.617726,VS0,VE5
Length: 296717 (290K) [application/java-archive]
Saving to: ‘org.osgi.core-6.0.0-sources.jar’

org.osgi.core-6.0.0 100%[=====================>] 289.76K  --.-KB/s   in 0.08s  

2016-05-02 11:21:06 (3.54 MB/s) - ‘org.osgi.core-6.0.0-sources.jar’ saved 
[296717/296717]
~~~~~~~~~~~~~~~~~~~~

~~ Ricardo