Added sort_mode and sortMode parameters to _geo_distance sort.

Previously it just support the "mode" parameter, which is
inconsistent

Closes elastic#3717

src/main/java/org/elasticsearch/search/sort/GeoDistanceSortParser.java

@@ -110,7 +110,7 @@ public SortField parse(XContentParser parser, SearchContext context) throws Exce
                 } else if ("normalize".equals(currentName)) {
                     normalizeLat = parser.booleanValue();
                     normalizeLon = parser.booleanValue();
-                } else if ("mode".equals(currentName)) {
+                } else if ("sort_mode".equals(currentName) || "sortMode".equals(currentName) || "mode".equals(currentName)) {
                     sortMode = SortMode.fromString(parser.text());
                 } else if ("nested_path".equals(currentName) || "nestedPath".equals(currentName)) {
                     nestedPath = parser.text();