SPARK-2181:The keys for sorting the columns of Executor page in SparkUI are incorrect

There must be a string, or compile error.
```
[error] /Users/witgo/work/code/java/spark/core/src/main/scala/org/apache/spark/ui/jobs/ExecutorTable.scala:78: overloaded method constructor UnprefixedAttribute with alternatives:
[error]   (key: String,value: Option[Seq[scala.xml.Node]],next: scala.xml.MetaData)scala.xml.UnprefixedAttribute <and>
[error]   (key: String,value: String,next: scala.xml.MetaData)scala.xml.UnprefixedAttribute <and>
[error]   (key: String,value: Seq[scala.xml.Node],next1: scala.xml.MetaData)scala.xml.UnprefixedAttribute
[error]  cannot be applied to (String, Long, scala.xml.MetaData)
[error]             <td sorttable_customekey={v.shuffleRead}>{Utils.bytesToString(v.shuffleRead)}</td>
[error]           
```