fix bug #67955

this fixes a regression from 6c2a8068207a02b3d7ae7416a9967dad0a81e61f.

smart_str_appendl is expecting the length as the length of the string,
but key_length is the byte length of the key, including the 0
terminator.

As such, the cookie name appeneded to the header would now also include
the 0 terminator of the key name which then would be sent to the server.
Fixed #67955: SoapClient prepends 0-byte to cookie names
Updated NEWS for #67955