Ignore additional MAPI challenge fields instead of raising an error

As long as the version number remains 9, unknown challenge fields can
safely be ignored.

The next release of MonetDB will send one such an additional field.

Author: joerivanruth

src/ServerChallenge.php

@@ -77,9 +77,9 @@ function __construct(string $challengeLine)
     {
         $parts = explode(':', $challengeLine);
         $count = count($parts) - 1;     	// Last is always empty
-        if ($count != 6) {
+        if ($count < 6) {
             throw new MonetException("Received invalid 'server challenge' string from the server. It"
-                ." contains {$count} fields, instead of 6.");
+                ." contains {$count} fields, should be at least 6.");
         }
 
         $this->salt = trim($parts[0]);