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

sorttable accepts the custom keys as HTML attributes, so they are untyped (they are strings). However, I think it tries to infer the data type so it will e.g. correctly covert them to integers.

The issue in the code here is that Scala's XML DSL only allows you to interpolate string expressions. So I think it's fine to call "toString" on these when we pass them to XML.