SolrParams ought to implement Iterable<Map.Entry<String,String[]>>

SolrJ ought to implement Iterable<Map.Entry<String,String[]>> so that it's easier to iterate on it, either using Java 5 for-each style, or Java 8 streams. The implementation on ModifiableSolrParams can delegate through to the underlying LinkedHashMap entry set. The default impl can produce a Map.Entry with a getValue that calls through to getParams.