time: time.Parse parses invalid dates

[austincb1]

go version go1.5.1 windows/amd64
Windows 7, cmd /v says Microsoft Windows [Version 6.1.7601]

time.Parse() parse invalid dates where it should report an error, for example: 31 Jun 2015 and 30 Feb 2015 are not valid.

Example in playground: http://play.golang.org/p/R4hQhgy24d

[bradfitz]

Intentional. See #12333.