Fractional second digit pattern handling is not exactly spec-compliant

[sffc]

The spec says:

Finally: If the requested skeleton included both seconds and fractional seconds and the dateFormatItem skeleton included seconds but not fractional seconds, then the seconds field of the corresponding pattern should be adjusted by appending the locale’s decimal separator, followed by the sequence of ‘S’ characters from the requested skeleton.

In other words, fractional second digits should he handled in a pattern as: h:mm:ss.SSS

However, we currently handle them without the decimal separator: h:mm:ssSSS

Unclear how/if/when this should be fixed. It's only an issue when power users give us patterns directly.