MARMOTTA-325 : Add hack to remove application/xml

There is no theoretical reason why we can't respond with application/xml
if they send application/xml, but in order to distinguish tuple and
graph results it may be easier just to remove it. This makes the
specific test that calls for application/sparql-results+xml as the
result for application/xml to succeed. Without the hack they get what
they asked for, application/xml
MARMOTTA-325 : Preference the default mime types before other mime types

OpenRDF add all of the common mime types for formats to the format
definition, including stating explicitly what the default should be.
This patch adds the default mime types to the LinkedHashSet first, so
they are iterated over before the other types.