net/http: set Content-Length:0 for empty PATCH requests as with POST, PATCH

Sets Content-Length:0 for nil bodies in PATCH requests, as we already do for  POST and PUT requests.

RFC 2616 mentions that unless a method’s Content-Length is forbidden it can send one.
In the wild, we’ve found that Microsoft Azure’s DataLake Gen2 storage API https://docs.microsoft.com/en-us/rest/api/storageservices/datalakestoragegen2/path/update deliberately rejects PATCH requests without a Content-Length, yet there is no workaround for setting that header when trying to flush the content of a file which was uploaded in a previous request.

Fixes #40978

Change-Id: Ib0a623b907d827a1c5ee431dca3c41024fa291c5
GitHub-Last-Rev: 12a3903f2bc22bcc4f5f8e2abcc3922b612b8871
GitHub-Pull-Request: golang/go#40991
Reviewed-on: https://go-review.googlesource.com/c/go/+/250039


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>