Fix Content-Length for POST when data empty

[rouja]

Q	A
Documentation	no
Bugfix	yes
BC Break	no
New Feature	no
RFC	yes
QA	no

Description

The RFC7230 says :

For example, a Content-Length header field is normally sent in a POST request even when the value is 0 (indicating an empty payload body).

Some load balancers reject empty POST without a Content-Lenght set to 0 with an HTTP 411 error. For instance Google Global Load Balancers does.