set default headers to application/xml in search

this was the previous behaviour and it is generally not recommended to
override those globals, however without that the search queries will
cease to work. this is because if ran behind apache, apache will set
Content-Type to be text/plain and then the underlying xml will not be
parsed

fix #1470, amend to #1474 
@dnahodil I know you like succinct comments, that's the shortest I could do to tell the story.