BUG: Series.align resets name when fill_value is specified
```
import pandas.util.testing as tm

ts = tm.makeTimeSeries()
ts.name = 'ts'
a, b = ts[2:], ts[:5]

# OK (names are preserved)
aa, ba = a.align(b)
aa.name
# ts
ba.name
# ts

# NG, specifying fill_value resets names
aa, ba = a.align(b, fill_value=-1)
aa.name
# None
ba.name
# None
```

@jreback Could you review this?
 lgtm
 Thanks!
