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 go-gitea#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 go-gitea#8030 and go-gitea#14532.

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.

Quick fix for imported dependency not used.

Co-authored-by: zeripath <[email protected]>