cpl_aws.cpp xml headers

[Kurt Schwehr]

In VSIS3HandleHelper::CanRestartOnError():

if( !STARTS_WITH(pszErrorMsg, "<?xml") )

From a discussion with a Google engineer:

Strictly speaking, there is well-formed XML this will not match, which either has characters preceding <?xml or has no <?xml at all.

It really ought to be matching the Content-Type header first before guessing the contents are XML based on content.

[warmerdam]

While this may be true in theory, does it happen in practice with the XML that is intended to be processed?

[Even Rouault]

In an attempt at lowering the number of opened tickets, closing that one. We might revisit the current approach if/when Amazon does a breaking change.