When making the following change, I haven't noticed that
g_httpuseragentfields contains a "Connection" header.
```
commit 092ce8144445a9b287554e59e3150bab3d67bedb
Author: YAMAMOTO Takashi <yamamoto@midokura.com>
Date: Mon Mar 7 09:30:23 2022 +0900
webclient: Always use "connection: close" for HTTP 1.1 for now
* This matches the HTTP 1.0 behavior.
* Persistent connection doesn't make much sense with the current API.
```
It seems that some servers are not happy with the duplicated header
and ignore them. (and do the default keep-alive for HTTP 1.1)
eg. Azure Blob (global)
f2737a208c