Editorial: PartitionDurationFormatPattern accessing non-existent slots

[ben-allen]

fix #190: PartitionDurationFormatPattern incorrectly tried to draw value of hours and seconds from DurationFormat object rather than from Duration Record.

[FrankYFTang]

LGTM

[FrankYFTang]

This PR will solve the accessing non-existent slots issue. However, it is not solving the output problem completely.
Consider the following input

{"hours":1,"minutes":0,"seconds":0,"milliseconds":1}
{"hours":1,"minutes":0,"seconds":0,"microseconds":1}
{"hours":1,"minutes":0,"seconds":0,"nanoseconds":1}

The output in the algorithm will not set displayRequired to true because duration.[[Seconds]] is 0.

[ben-allen]

marking this one as closed, since #188 fixed both:

• the invalid slot access issue
• the issue with seconds being marked as not formatted when seconds is 0 but there exists a subsecond unit with a nonzero value.