pygmt.config: pen settings are not reverted correctly when set in a context manager

Haven't found time to see why it doesn't work.

Here is the minimal example:
```python
import pygmt

fig = pygmt.Figure()

fig.basemap(region=[0, 10, 0, 10], projection="X10c", frame=True)

fig.shift_origin(xshift="w+1")
with pygmt.config(MAP_FRAME_PEN="4p,blue"):
fig.basemap(region=[0, 10, 0, 10], projection="X10c", frame=True)

fig.shift_origin(xshift="w+1")
fig.basemap(region=[0, 10, 0, 10], projection="X10c", frame=True)
fig.show()
```
The 3rd basemap is expected to have the default frame pen setting. Now the pen color is correct, but the pen thickness is incorrect:

| Actual | Expected |
|---|---|
|  |  |

The expected image is produced by:
```
gmt begin map
gmt basemap -R0/10/0/10 -JX10c -Baf
gmt basemap -R0/10/0/10 -JX10c -Baf -Xw+1c --MAP_FRAME_PEN=3p,blue
gmt basemap -R0/10/0/10 -JX10c -Baf -Xw+1c
gmt end show
```