Unicode list cast fix

Fixes this: http://ci.openquake.org/job/oq-engine/317/testReport/junit/tests.calculators.risk.classical.core_test/PreExecuteTestCase/test_pre_execute_check_imts_raises/

Basically, the problem is that we were casting a `list` as a `str`, which in some environments  would appear as `[u'PGA']` and others `['PGA']`.