fix longitude_180 trouble (second edition)

I check this code by run this:

```
(-90..90).each do |t|
  a = GeoHash.encode(t, 180, 12)
  b = GeoHash.adjacent(a, :right)
  raise if b != GeoHash.encode(t, -180, 12)
end
```
and
```
(-90..90).each do |t|
  a = GeoHash.encode(t, -180, 12)
  b = GeoHash.adjacent(a, :left)
  raise if b != GeoHash.encode(t, 180, 12)
end
```