Apply highlighting to hl.alternateField

Today, you can configure hl.alternateField for highlighter to display if no snippets were produced from original field. But the contents of the fallback field is output without highlighting the original query terms.

This issue will cause alternate field to be highlighted with no snippet generation, and still respect max length. You can turn it off using new param hl.highlightAlternate=false. Supported highlighters: Simple, FVH