fix case where colorspecs mishandled serializing to dictionaries when the value was unset

The problem stemmed from using `self._isset` to distinguish between "never having been set" and "explicitly set to None". By ignoring `self.value` the color spec would always serialize to `None` whenever it had been set. The was exposed when running the glyph tests after using the `plotting.py` interface:
```
In [1]: %run test_glyphs.py
............................................................................................
----------------------------------------------------------------------
Ran 92 tests in 0.375s

OK

In [2]: from bokeh.plotting import circle

In [3]: circle([1,2], [3,4])
Out[3]: <bokeh.objects.Plot at 0x1104b8c10>

In [4]: %run test_glyphs.py
..F..F..F.F......F..F.F.F.F.F.F..........F..F..F..F..F..F..F..F..F..F..F..F..F.F.F....F..F.F
```
As of this change, all existing tests pass, a new `test_named_value_unset` test passes. Visual inspection of all test .png outputs also looks correct. However, being a change at a very low level, closer scrutiny is requested from @pzwang @mattpap @damianavila