Issue #4860 NPE from HttpFields

+ Fix bug with list iterator nextIndex
 + List iterator cannot inject null fields
 + minor cleanups

Signed-off-by: Greg Wilkins <gregw@webtide.com>

jetty-http/src/main/java/org/eclipse/jetty/http/HttpFields.java

@@ -303,14 +303,18 @@ public class HttpFields implements Iterable<HttpField>
      */
     public List<String> getValuesList(String name)
     {
-        final List<String> list = new ArrayList<>();
+        List<String> list = null;
         for (int i = 0; i < _size; i++)
         {
             HttpField f = _fields[i];
             if (f.getName().equalsIgnoreCase(name))
+            {
+                if (list == null)
+                    list = new ArrayList<>(size() - i);
                 list.add(f.getValue());
             }
-        return list;
+        }
+        return list == null ? Collections.emptyList() : list;
     }
 
     /**
@@ -711,6 +715,7 @@ public class HttpFields implements Iterable<HttpField>
 
     public void add(HttpHeader header, HttpHeaderValue value)
     {
+        if (value != null)
             add(header, value.toString());
     }
 
@@ -966,8 +971,11 @@ public class HttpFields implements Iterable<HttpField>
      *
      * @param fields the fields to add
      */
+    @Deprecated
     public void add(HttpFields fields)
     {
+        // TODO this implementation doesn't do what the javadoc says and is really the same
+        // as addAll, which is renamed to add anyway in 10.
         if (fields == null)
             return;
 
@@ -1185,7 +1193,7 @@ public class HttpFields implements Iterable<HttpField>
         @Override
         public int nextIndex()
         {
-            return _cursor + 1;
+            return _cursor;
         }
 
         @Override
@@ -1199,11 +1207,16 @@ public class HttpFields implements Iterable<HttpField>
         {
             if (_current < 0)
                 throw new IllegalStateException();
+            if (field == null)
+                remove();
+            else
                 _fields[_current] = field;
         }
 
         @Override
         public void add(HttpField field)
+        {
+            if (field != null)
             {
                 _fields = Arrays.copyOf(_fields, _fields.length + 1);
                 System.arraycopy(_fields, _cursor, _fields, _cursor + 1, _size++);
@@ -1212,3 +1225,4 @@ public class HttpFields implements Iterable<HttpField>
             }
         }
     }
+}