PDFDataset does not use .aux.xml files

[oleinik]

and as result does not save/load users metadata.

class PDFDataset described as descendant from class GDALPamDataset. A subclass of GDALDataset which introduces the ability to save and restore auxiliary information (coordinate system, gcps, metadata, etc) not supported by a file format via an "auxiliary metadata" file with the .aux.xml extension.

PDFDataset has call to TryLoadXML, but next calls from client program to GDALGetMetadataItem does not returns any values saved in corresponding .aux.xml file. After changing metadata through GDALSetMetadataItem for users metadata domain and call to GDALFlushCache .aux.xml file does not creating or rewriting and all modifications are lost.

[Even Rouault]

In 34733:

PDF: implement loading/saving of metadata from/into PAM (fixes #6600)

[Even Rouault]

In 34734:

Fix test failure on Travis (fixes #6600)