Metadata is properly encoded

Special characters (like agnles, ampersand, or quotes) in metadata should be properly encoded,
or result will be invalid (X)HTML. Previous version passes metadata into result as-is. This change
applies encoding.

Existing test XHTML_Headers is modified to use few special characters in metadata.

All tests pass.
Merge pull request #22 from van-de-bugger/master

Metadata is properly encoded