Add Content-Length header to HEAD requests (#14542)

Add Content-Length header to HEAD requests (#14542)

* Add Content-Length header to HEAD requests

This change adds the header Content-Length to HEAD HTTP requests.

The previous behaviour was blocking some Windows executables (i.e
bitsadmin.exe) from downloading files hosted in Gitea.

This along with PR #14541, makes the web server compliant with HTTP RFC 2616 which states
"The methods GET and HEAD MUST be supported by all general-purpose servers"
and
"The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response."

This should also respond to issues #8030 and #14532.

* This change adds the header Content-Length to HEAD HTTP requests

Pass the Size of the content as a parameter to ServeData() instead of
calculating it using ioutil.ReadAll(reader) --> this call is dangerous
and can result in a denial of service.

* Add Content-Length header to HEAD requests

Quick fix for imported dependency not used.

* Check if size is positiv int ...

Co-authored-by: zeripath <art27@cantab.net>