Fix call to undefined method Field::append()

src/HAB/Pica/Record/AuthorityRecord.php

@@ -83,7 +83,7 @@ class AuthorityRecord extends Record
             if ($ppnSubfield) {
                 $ppnSubfield->setValue($ppn);
             } else {
-                $ppnField->append(new Subfield('0', $ppn));
+                $ppnField->addSubfield(new Subfield('0', $ppn));
             }
         } else {
             $this->append(new Field('003@', 0, array(new Subfield('0', $ppn))));

src/HAB/Pica/Record/CopyRecord.php

@@ -120,7 +120,7 @@ class CopyRecord extends Record
             if ($epnSubfield) {
                 $epnSubfield->setValue($epn);
             } else {
-                $epnField->append(new Subfield('0', $epn));
+                $epnField->addSubfield(new Subfield('0', $epn));
             }
         } else {
             $this->append(new Field('203@', $this->getItemNumber(), array(new Subfield('0', $epn))));

src/HAB/Pica/Record/TitleRecord.php

@@ -183,7 +183,7 @@ class TitleRecord extends NestedRecord
             if ($ppnSubfield) {
                 $ppnSubfield->setValue($ppn);
             } else {
-                $ppnField->append(new Subfield('0', $ppn));
+                $ppnField->addSubfield(new Subfield('0', $ppn));
             }
         } else {
             $this->append(new Field('003@', 0, array(new Subfield('0', $ppn))));