http: when unfolding, leave single-space for new header line

Restore the unfolding behavior from before 67ae101666. This change
(leaving more whitespace in the delivered headers) turned out causing
some friction in the git project so presumably others might also find it
a little surprising.

Reported-by: Jeff King
Ref: https://marc.info/?l=git&m=176606332701171&w=2
Closes #20016

tests/data/test1274

@@ -55,7 +55,7 @@ Server: test-server/ fake folded
 Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
 ETag: "21025-dc7-39462498"
 Content-Length: 6
-Connection:%repeat[46 x  ]%   close
+Connection: close
 
 </file>
 </verify>

tests/data/test1940

@@ -59,7 +59,7 @@ http://%HOSTIP:%HTTPPORT/%TESTNUMBER
 - Set-Cookie == onecookie=data; (0/3)
 - Set-Cookie == secondcookie=2data; (1/3)
 - Set-Cookie == cookie3=data3; (2/3)
- Fold == is folding a        line
+ Fold == is folding a line
  Blank ==%SP
  Blank2 ==%SP
 </stdout>