check attributes in XTS class

use attr function in order to check attributes in XTS class.

In the case below, all XTS timezone attribute is set to "UTC", while system timezone is NULL.

> attr(GDP_XTS,'tz')
[1] "UTC"
> attr(GDPC96,'tz')
[1] "UTC"
> attr(GDP,'tz')
[1] "UTC"
> attr(RRSFS,'tz')
[1] "UTC"
> Sys.getenv("TZ")
[1] ""