Decode: preserve nanosecond precision when decoding time

[moorereason]

Updates #613
Competitor to #621

[pelletier]

Note for later: I'd expect this to be a || instead of a &&, so that when Precision is 0 the nanoseconds are not displayed. However this does create an issue where the zero-value of LocalTime is has the surprising behavior:

t := LocalTime{Nanosecond: 42}
t.String() => "00:00:00"

[moorereason]

My primary use case was decoding. I was trying to allow for this scenario (which I failed to cover in this PR):

diff --git a/localtime_test.go b/localtime_test.go
index 089a8ba..9c9b681 100644
--- a/localtime_test.go
+++ b/localtime_test.go
@@ -41,6 +41,8 @@ func TestLocalTime_String(t *testing.T) {
        require.Equal(t, "20:12:01.000000002", d.String())
        d = toml.LocalTime{20, 12, 1, 0, 0}
        require.Equal(t, "20:12:01", d.String())
+       d = toml.LocalTime{20, 12, 1, 0, 9}
+       require.Equal(t, "20:12:01.000000000", d.String())
 }

 func TestLocalTime_MarshalText(t *testing.T) {
diff --git a/unmarshaler_test.go b/unmarshaler_test.go
index e7e44f7..0fddbbc 100644
--- a/unmarshaler_test.go
+++ b/unmarshaler_test.go
@@ -2038,6 +2038,11 @@ func TestLocalDateTime(t *testing.T) {
                input string
                prec  int
        }{
+               {
+                       desc:  "9 digits zero nanoseconds",
+                       input: "2006-01-02T15:04:05.000000000",
+                       prec:  9,
+               },
                {
                        desc:  "9 digits",
                        input: "2006-01-02T15:04:05.123456789",

I can push these test additions up if you want me to.

I hadn't completely thought through the case you offered. If the user plans to have custom LocalTime values, they will need to also set Precision in order for the String() output to be what's expected. Is this a use case we need to be concerned about? I assumed that most people will be using time.Time values in their app instead of toml.LocalTime.

[moorereason]

I went ahead and pushed up these additional test cases.

[pelletier]

Ended up going with a mix of both ideas by providing the minimum number of digits as precision when it is not specified d58efa3.