The previous `Fprintf` didn't have a format string. That means that it writes a string like this to the summer:

    bar%!(EXTRA string=)1961029597390627509

This is probably not what we wanted, although it does produce an acceptable hash by accident.

With the fixed version, if there is no delimiter, "foo"="bar" will hash to the same as "fo"="obar". The 0-byte prevents this.