replaced boolean search params with string according to discogs docs

[schnubor]

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.

[ricbra]

I made a mistake there. The've updated the API docs pages lately, now all params are better documented. I thought these were booleans to trigger the search of q.

Thanks for your PR, could you also fix the search test?

[schnubor]

Yep, the new docs are much better now. Fixed the test, sorry :)

[ricbra]

Thanks @schnubor