Content Length Header is not preserved for HEAD requests

#### Which platform are you running AnyProxy
Mac

#### The version of the AnyProxy
4.0.6

#### Your expected behavior of AnyProxy
When Proxying HEAD requests, the content-length header should be preserved unless modified by a hook

#### The actual behavior of AnyProxy
Content Length is set to 0 as the body is an empty buffer.

Start by tracing how AnyProxy handles HEAD requests and calculates the Content-Length header, then reproduce the reported behavior on the NodeJS proxy path. Done means a proxied HEAD request preserves its original Content-Length unless a hook changes it; add or update coverage if the relevant test location is found.

Written by the indexing model from the issue text.