Clear attributes in incrementToken to fix IllegalArgumentException.

We ran into IllegalArgumentException for an uninitialized position increment. Adding clearAttributes call to incrementToken solved this problem for us.

    java.lang.IllegalArgumentException: Increment must be zero or greater: -1873026560 
    at org.apache.lucene.analysis.tokenattributes.PackedTokenAttributeImpl.setPositionIncrement(PackedTokenAttributeImpl.java:50) 
    at org.apache.lucene.analysis.util.FilteringTokenFilter.incrementToken(FilteringTokenFilter.java:58) 
    at uk.co.flax.luwak.presearcher.TermFilteredPresearcher.buildQuery(TermFilteredPresearcher.java:80) 
    at uk.co.flax.luwak.Monitor.buildQuery(Monitor.java:174) 
    at uk.co.flax.luwak.Monitor.match(Monitor.java:162) 
    at uk.co.flax.luwak.Monitor.match(Monitor.java:189) 
    ...