replaced boolean search params with string according to discogs docs

Providing string params for 'artist', 'title' and 'format' in search gave me errors so I noticed the majority of params were actually declared as booleans. I guess that shouldn't be the case. According to Discogs every param should be a string:

http://www.discogs.com/developers/#page:database,header:database-search

tested 'artist', 'title' and 'format' as string again and it worked. Didn't doublecheck the other params though.