Fixes required due to PR69 changes to AbstractID3v2Tag.getFrame() so that it always returns list

Issue #349 resolved
IJabz repo owner created an issue

AbstractID3v2Tag.getFrame() used to return a frame if only single frame for id, or a list if multiple, but this was a bit messy.

So since 3.0.0 now always returns a list even if only one item (the norm), but we missed a couple of changes required that werent covered by tests.