TypeError: WritableUtils.writeString is not a function

When setting an attribute for scan using `scan.setAttribute(name, value)`, causing the following error thrown.

Seems like there is no method called `writeString` in https://github.com/alibaba/node-hbase-client/blob/04463814fd89db6e972ce904d592ccd9ea16947a/lib/writable_utils.js

Hbase client version: 1.4.0

```
2017-03-22 18:07:13,948 ERROR 151285 nodejs.TypeError: WritableUtils.writeString is not a function (uncaughtException throw 1 times on pid:151285)
at Scan.OperationWithAttributes.writeAttributes (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/operation_with_attributes.js:63:21)
at Scan.write (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/scan.js:170:8)
at Object.exports.writeObject (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/io/hbase_object_writable.js:339:14)
at Invocation.write (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/ipc/invocation.js:52:25)
at Connection.sendParam (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/connection.js:238:14)
at Connection.call (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/connection.js:371:8)
at Connection.openScanner (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/connection.js:466:8)
at Client. (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/client.js:208:21)
at Client.g (events.js:291:16)
at emitTwo (events.js:106:13)
at Client.emit (events.js:191:7)
at /home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/client.js:980:12
at Call. (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/connection.js:359:5)
at emitTwo (events.js:106:13)
at Call.emit (events.js:191:7)
at Call.callComplete (/home/admin/fsp/target/fsp/node_modules/.1.4.0@hbase-client/lib/connection.js:554:8)
```

Start with lib/writable_utils.js and lib/operation_with_attributes.js, then trace the write path through lib/scan.js where the stack trace reports the failure. Reproduce a scan using scan.setAttribute(name, value) and verify that writing the scan completes without the reported TypeError.

Written by the indexing model from the issue text.