corrupted utf-8 XML output files

[darkblueB]

converting a shape file to GML or KML may result in corrupted utf-8 encoding. The xml header for GML or KML will claim utf-8.

As per a discussion on IRC, a simple step toward file integrity is to ensure that string data input is not changed upon output. (If the result of a conversion is known to produce invalid utf-8 XML, then perhaps a switch on the command line to turn off the indication in the header in the output file would be appropriate) At any rate, the current situation can result in problematic files with no warnings.

[Even Rouault]

See #2971 that forces XML output to be ASCII if it is not valid UTF-8.