Set the head table checkSumAdjustment to zero

...before calculating the head table checksum. Per the head table specification:

> checkSumAdjustment: To compute set it to 0, calculate the checksum for the 'head' table and put it in the table directory, sum the entire font as a uint32_t, then store 0xB1B0AFBA - sum. (The checksum for the 'head' table will be wrong as a result. That is OK; do not reset it.)

ref #123

src/FontLib/Table/Type/head.php

@@ -43,4 +43,9 @@ protected function _parse() {
       throw new Exception("Incorrect magic number (" . dechex($this->data["magicNumber"]) . ")");
     }
   }
+
+  function _encode() {
+    $this->data["checkSumAdjustment"] = 0;
+    return parent::_encode();
+  }
 }