fdsn: fix wadl parser, usgs test actually never ever passed since being added ;)

 - dicts aren't ordered (and who said the default namespace must come
   first in the xml attribute list?), not the default but a random
   namespace was picked as the default
 - much more robust xpath fix with default namespaces, would have
   failed in case of a mix of default and other namespaces in the
   expression (see
   https://github.com/barsch/seishub.core/pull/4/files#diff-3309b0bd72b4a630e196507be23e0e0cR11)
fdsn: fix a test that fails due to an issue with IRIS event WADL, should
be removed again, when IRIS updates the event WADL (and after updating
data/event.wadl)
fdsn: fix failing tests

tests were running fine when executing the corresponding test. tests
were only failing with `obspy-runtests fdsn` because of prior warnings.