More Like This Handler uses only first field in mlt.fl when using stream.body

The documentation at: http://wiki.apache.org/solr/MoreLikeThisHandler indicates that one can use multiple fields for similarity in mlt.fl:
http://localhost:8983/solr/mlt?stream.body=electronics%20memory&mlt.fl=manu,cat&mlt.interestingTerms=list&mlt.mintf=0

In trying this, only one field is used.
Looking at the code, it only looks at the first field:

public DocListAndSet getMoreLikeThis( Reader reader, int start, int rows, List<Query> filters, List<InterestingTerm> terms, int flags ) throws IOException
{
// analyzing with the first field: previous (stupid) behavior
rawMLTQuery = mlt.like(reader, mlt.getFieldNames()[0]);