bfastts: error if time series contains leap day

If we have the leap day (Feb 29) and also the day after (March 1) in the time series, `bfastts` fails with an error:
```
Error in merge.zoo(zoo(coredata(x), tt), zoo(, tt2)) :
series cannot be merged with non-unique index entries in a series
In addition: Warning messages:
1: In zoo(data, 1900 + as.POSIXlt(dates)$year + (yday365(dates) - 1)/365, :
some methods for “zoo” objects do not work if the index entries in ‘order.by’ are not unique
```

MWE:
Fails:
```r
a = c(as.Date("2016-02-29"), as.Date("2016-03-01"))
b = 1:2
bfastts(b, a, type="irregular")
```
Works if we set it to be one day later:

```r
a = c(as.Date("2016-02-29"), as.Date("2016-03-01"))
b = 1:2
bfastts(b, a+1, type="irregular")
```