to.monthly, to.quarterly drop index tzone attribute

[joshuaulrich]

R> x <- xts(1:2, as.Date(c("2015-12-01", "2016-01-01")))
R> unlist(attributes(.index(x)))
 tzone tclass 
 "UTC" "Date" 
R> unlist(attributes(.index(to.weekly(x))))
tzone.TZ   tclass 
   "UTC"   "Date" 
R> unlist(attributes(.index(to.monthly(x))))
   tclass 
"yearmon" 
R> unlist(attributes(.index(to.quarterly(x))))
   tclass 
"yearqtr" 
R> unlist(attributes(.index(to.yearly(x))))
tzone.TZ   tclass 
   "UTC"   "Date" 

The issue manifests inside xts:::.drop.time when indexClass is yearmon or yearqtr. The tzone attribute is present until the index(x) <- index(x) line in .drop.time.