It is legal to have index key specification to be simple string. (#124)

So far, we are expecting it to be object, Mongo shell can provide key
spec as simple string for simple indexes.

Author: apkar

src/ExtMsg.actor.cpp

@@ -753,10 +753,28 @@ ACTOR Future<WriteCmdResult> attemptIndexInsertion(bson::BSONObj indexObj,
                                                    Reference<ExtConnection> ec,
                                                    Reference<DocTransaction> tr,
                                                    Namespace ns) {
-
 	if (!indexObj.hasField(DocLayerConstants::NAME_FIELD))
 		throw no_index_name();
 
+	// It is legal to have index key to be simple string for simple value indexes.
+	if (indexObj.getField(DocLayerConstants::KEY_FIELD).isString()) {
+		bson::BSONObjBuilder builder;
+		for (auto it = indexObj.begin(); it.more();) {
+			auto el = it.next();
+			if (std::string(el.fieldName()) == DocLayerConstants::KEY_FIELD) {
+				builder.append(DocLayerConstants::KEY_FIELD, BSON(el.String() << 1));
+			} else {
+				builder.append(el);
+			}
+		}
+		indexObj = builder.obj();
+	}
+
+	if (!indexObj.getField(DocLayerConstants::KEY_FIELD).isABSONObj()) {
+		TraceEvent(SevWarn, "BadIndexSpec").detail("err_msg", "Bad key format");
+		throw bad_index_specification();
+	}
+
 	if (indexObj.getObjectField(DocLayerConstants::KEY_FIELD).nFields() == 1) {
 		auto keyEl = indexObj.getObjectField(DocLayerConstants::KEY_FIELD).firstElement();
 		if (!keyEl.isNumber() || !(keyEl.Number() == 1.0 || keyEl.Number() == -1.0)) {