`logical_or` does not deal with default values correctly

Consider the following piece of code:

```
>>> a = traces.TimeSeries()
>>> a.default = 1
>>> a[10] = 0
>>> b = traces.TimeSeries()
>>> b.default = 0
>>> c = a.logical_or(b)
>>> a[1]
1
>>> b[1]
0
>>> c[1]
0
>>>
```