BUGFIX: Removed XHTML XML declaration requirement

An XML declaration is not required in all XML documents; however XHTML document authors are strongly encouraged to use XML declarations in all their documents. Such a declaration is required when the character encoding of the document is other than the default UTF-8 or UTF-16 and no encoding was determined by a higher-level protocol.

Discussion here: https://groups.google.com/forum/?fromgroups=#!topic/silverstripe-dev/akRYY5ev6w0