UnifiedHighlighter can highlight terms within SpanNear clauses at unmatched positions

The UnifiedHighlighter (and original Highlighter) highlight phrases by converting to a SpanQuery and using the Spans start and end positions to assume that every occurrence of the underlying terms between those positions are to be highlighted. But this is inaccurate; see LUCENE-5455 for a good example, and also LUCENE-2287. The solution is to use the SpanCollector API which was introduced after the phrase matching aspects of those highlighters were developed.