[SPARK-15766][SPARKR] R should export is.nan

## What changes were proposed in this pull request?

When reviewing SPARK-15545, we found that is.nan is not exported, which should be exported.

Add it to the NAMESPACE.

## How was this patch tested?

Manual tests.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #13508 from wangmiao1981/unused.

R/pkg/NAMESPACE

@@ -185,6 +185,8 @@ exportMethods("%in%",
               "isNaN",
               "isNotNull",
               "isNull",
+              "is.nan",
+              "isnan",
               "kurtosis",
               "lag",
               "last",