Data.frame is not supported when first columns is a date.time


```R
> dygraph(data.frame(date = as.Date(Sys.time()), metric = 1))
Error in dygraph(data.frame(date = as.Date(Sys.time()), metric = 1)) : 
  Unsupported type passed to argument 'data'.
> dygraph(data.frame(date = (Sys.time()), metric = 1))
Error in dygraph(data.frame(date = (Sys.time()), metric = 1)) : 
  Unsupported type passed to argument 'data'.
``` 

