* 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