Serialize bucket keys as strings as opposed to optional strings.

Author: jtibshirani

server/src/main/java/org/elasticsearch/search/aggregations/bucket/range/InternalBinaryRange.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.search.aggregations.bucket.range;
 
 import org.apache.lucene.util.BytesRef;
+import org.elasticsearch.Version;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.xcontent.XContentBuilder;
@@ -73,17 +74,25 @@ private static String generateKey(BytesRef from, BytesRef to, DocValueFormat for
         }
 
         private static Bucket createFromStream(StreamInput in, DocValueFormat format, boolean keyed) throws IOException {
-            String key = in.readOptionalString();
+            String key = in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)
+                ? in.readString()
+                : in.readOptionalString();
+
             BytesRef from = in.readBoolean() ? in.readBytesRef() : null;
             BytesRef to = in.readBoolean() ? in.readBytesRef() : null;
             long docCount = in.readLong();
             InternalAggregations aggregations = InternalAggregations.readAggregations(in);
+
             return new Bucket(format, keyed, key, from, to, docCount, aggregations);
         }
 
         @Override
         public void writeTo(StreamOutput out) throws IOException {
-            out.writeOptionalString(key);
+            if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+                out.writeString(key);
+            } else {
+                out.writeOptionalString(key);
+            }
             out.writeBoolean(from != null);
             if (from != null) {
                 out.writeBytesRef(from);