Fix the return type of WriteBatch::Data().

Summary: Quick fix for https://reviews.facebook.net/D15123

Test Plan: Make check

Reviewers: sdong, vkrest

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15165

include/rocksdb/write_batch.h

@@ -88,7 +88,7 @@ class WriteBatch {
   Status Iterate(Handler* handler) const;
 
   // Retrieve the serialized version of this batch.
-  std::string Data() const { return rep_; }
+  const std::string& Data() const { return rep_; }
 
   // Retrieve data size of the batch.
   size_t GetDataSize() const { return rep_.size(); }