time: include const for time.Time.String()'s layout

[gopherbot]

by michael.schurter:

The layout used by the time.Time.String() method is hardcoded into the method. If it
were a constant it would be easier to roundtrip time.Time's through a string (or []byte)
without having to copy and paste the layout out of Time.String's docstring.

Relevant code: https://code.google.com/p/go/source/browse/src/pkg/time/format.go#397

[robpike]

Comment 1:

Status changed to Started.

[robpike]

Comment 2:

The default format string is a clumsy one whose sole merit is that it represents all the
pieces of the value. Few uses need that property alone. A better approach is to choose a
format that meets your needs (say, RFC3999Nano) and use that explicitly. The only reason
to publish this as a constant is to parse a default value again. If you're parsing, it's
better to be clear about what you're printing too.

Status changed to WorkingAsIntended.