Pie: Fix combined data to keep all other original values.

But not only data, color, label, angle, percent as was before.
Allows to store own values in series and use them e.g. in labelFormatter.
Add a comment explaining why extend is being used
Merge pull request #1238 from SaM-Solutions/fix-combined-data

Pie: Fix combined data to keep all other original values.
Merge pull request #1 from flot/master

update