Added a clear method for WriteBatch

src/db.rs

@@ -1223,6 +1223,14 @@ impl WriteBatch {
             Ok(())
         }
     }
+
+    /// Clear all updates buffered in this batch.
+    pub fn clear(&mut self) -> Result<(), Error> {
+        unsafe {
+            ffi::rocksdb_writebatch_clear(self.inner);
+        }
+        Ok(())
+    }
 }
 
 impl Default for WriteBatch {

tests/test_write_batch.rs

@@ -0,0 +1,27 @@
+// Copyright 2019 Tyler Neely
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+extern crate rocksdb;
+
+use rocksdb::WriteBatch;
+
+#[test]
+fn test_write_batch_clear() {
+    let mut batch = WriteBatch::default();
+    let _ = batch.put(b"1", b"2");
+    assert_eq!(batch.len(), 1);
+    let _ = batch.clear();
+    assert_eq!(batch.len(), 0);
+    assert!(batch.is_empty());
+}