Search for exact citekey

Hi,
I'm not sure if this behaviour is intented or not. We want to include references to single papers on a research page, so we put a lot of biblio_db_search('citekey','author_2008') into the code. Unfortunately, this finds all papers thats citekey starts with author_2008, i.e., also author_2008a, author_2008b etc.
We'd prefer to find only the paper whose citekey matches exactly. The citekey keywords is caught by the default case and therefore handled with RLIKE at the moment.

If the rlike-search isn't desired for citekeys for any special reason that I miss, feel free to apply the attached patch to change the behaviour.