Porting LTR plugin for Solr-5.5.0

[Ahmet Anil Pala]

Hi, I need to use the LTR plugin which compiles for Solr-6.0.0 
[here](https://github.com/bloomberg/lucene-solr/tree/master-ltr-plugin-rfc-cpoerschke-comments)

I have attempted to port the plugin for Solr-5.5.0 
[here](https://github.com/aanilpala/lucene-solr/commit/94ad14c4b9eae2c899e3941967f59b9fc20401b9)

Reranking works fine, however when I want to extract features with the 
following parameters, I get an NPE : fv=true&fl=*,[features]`
Here is the stack trace :

```
o.a.s.s.HttpSolrCall null:java.lang.NullPointerException
        at 
org.apache.solr.ltr.ranking.LTRFeatureLoggerTransformerFactory$FeatureTransformer.transform(LTRFeatureLoggerTransformerFactory.java:131)
        at 
org.apache.solr.response.transform.DocTransformers.transform(DocTransformers.java:76)
        at org.apache.solr.response.DocsStreamer.next(DocsStreamer.java:160)
        at 
org.apache.solr.response.TextResponseWriter.writeDocuments(TextResponseWriter.java:246)
        at 
org.apache.solr.response.TextResponseWriter.writeVal(TextResponseWriter.java:151)
        at 
org.apache.solr.response.JSONWriter.writeNamedListAsMapWithDups(JSONResponseWriter.java:183)
        at 
org.apache.solr.response.JSONWriter.writeNamedList(JSONResponseWriter.java:299)
        at 
org.apache.solr.response.JSONWriter.writeResponse(JSONResponseWriter.java:95)
        at 
org.apache.solr.response.JSONResponseWriter.write(JSONResponseWriter.java:60)
        at 
org.apache.solr.response.QueryResponseWriterUtil.writeQueryResponse(QueryResponseWriterUtil.java:52)
        at 
org.apache.solr.servlet.HttpSolrCall.writeResponse(HttpSolrCall.java:743)
        at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:467)
        at 
org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:225)
        at 
org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:183)
        at 
org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
        at 
org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
        at 
org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
        at 
org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
        at 
org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
        at 
org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
        at 
org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
        at 
org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
        at 
org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
        at 
org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
        at 
org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:215)
        at 
org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:110)
        at 
org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
        at org.eclipse.jetty.server.Server.handle(Server.java:499)
        at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:310)
        at 
org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)
        at 
org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:540)
        at 
org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
        at 
org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
        at java.lang.Thread.run(Thread.java:745)
```
Regards
Anil