fix list submission for isrcs, puids and echoprints

A bug was introduced with
291025c25d8903848bb01d90e36d1fd8689ceaf7 (in version 0.4):
Isrcs were always enclosed in [], even if they were already given as a
list.
So in 0.3 only giving isrcs as lists works, see #104
and in 0.4 only giving isrcs as single isrc works.

Now both should actually work.