as_datetime("2017-01-20") returns NA, breaks compatibility

[mmitkevich]

Hi, latest as_datetime doesn't parse ISO time string any more.

> lubridate::as_datetime("2017-01-20")
[1] NA
Warning message:
All formats failed to parse. No formats found. 

I had to insert old implementation

setMethod("as_datetime", "character",
          function(x, tz = "UTC") {
            with_tz(as.POSIXct(x), tzone = tz)
          })

into my code to revert to old semantics. It breaked my code.

Why shouldn't as_datetime parse dates without time as midnights?

[vspinu]

Hm. This breaking change slipped my radar. Consequence of #536 and #527. We want a consistent behavior of all of the parsing code. Will fix in a moment.