Minimal flux fix for CandleStickSeries regression from Attribute perf changes.

BACKGROUND

Prior to the Attribute perf changes, we used `state.addAll()` to set up
the initial config for an attribute while adding it. For performance
reasons this was replaced by simply setting `state.data` directly, to
consciously avoid iteration and mix costs for the initial add.

However there are use cases, such as CandleStickSeries' relationship
between it's `graphic` and `upcandle` attributes where this causes
a regression.

`graphic` is defined by a superclass, but it's setter is overridden by
CandleStickSeries. The overridden setter sets the `upcandle` attribute,
which only exists on CandleStickSeries, not on the superclass.

As a result, the value of `upcandle` is set while the `graphic` setter
is executed, before the `upcandle` attribute is formally added.

In older versions, we'd add the `upcandle` attribute later, when
processing CandleStickSeries attributes, and since we just merged,
we'd maintain the original set value. In the new version, since we
set `state.data` directly, the original value gets blown away.

FIX

The localized fix, aims to re-introduce the 'merge' behavior, if
a value already exists, while still trying to keep the optimized
performance.

The longer term fix, which is targetted for the next release is
to remove the class-by-class attribute setup and just added all
attributes in one shot, there by avoiding the issue.

TESTING

All library tests pass through yogi --no-cli. A unit test was added
to attribute for this use case.

Total]: Passed: 10843 Failed: 0 Total: 10936 (ignored 93)
Merge branch 'attrs-subclass-setter-override-fix' into dev-3.x
Build for Attribute/CandleStickSeries regression fix.

https://github.com/yui/yui3/pull/626