Fix single quote parsing of TOML durations

closes #2023

internal/internal_test.go

@@ -142,6 +142,10 @@ func TestDuration(t *testing.T) {
 	d.UnmarshalTOML([]byte(`1s`))
 	assert.Equal(t, time.Second, d.Duration)
 
+	d = Duration{}
+	d.UnmarshalTOML([]byte(`'1s'`))
+	assert.Equal(t, time.Second, d.Duration)
+
 	d = Duration{}
 	d.UnmarshalTOML([]byte(`10`))
 	assert.Equal(t, 10*time.Second, d.Duration)