* Fix for missing content-length header when using QUIC
If request.ContentLength is set then it will be used instead of getting
it from request.Header map since quic-go(lucas-clemente/quic-go@bb24be8)
will not store (and pass) the Content-Length header using its header
map.
This fixes a potential issue where FastCGI POST requests body empty when
QUIC is enabled. (#1370)
* Change the data type for fastcgi contentLength to int64
quic-go uses int64 for contentLength
* Fix an error for undeclared variable
* Fix test for fcgiclient
the data type for contentLength
* Convert rwc field on FCGIClient from type io.ReadWriteCloser to net.Conn.
* Return HTTP 504 to the client when a timeout occurs.
* In Handler.ServeHTTP(), close the connection before returning an HTTP
502/504.
* Refactor tests and add coverage.
