[BUG] checks: http-check expect could fail a check on multi-packet responses

[BUG] checks: http-check expect could fail a check on multi-packet responses

Christopher Blencowe reported that the httpchk_expect() function was
lacking a test for incomplete responses : if the server sends only the
headers in the first packet and the body in a subsequent one, there is
a risk that the check fails without waiting for more data. A failure
rate of about 1% was reported.

This fix must be backported to 1.4.