pygmt.grdtrack: Wrong column names of output pandas DataFrame

### Description of the problem

_This is related to the GMT forum post at_ https://forum.generic-mapping-tools.org/t/type-error-with-pygmt-grdtrack-possible-bug-or-intended/4286.

If the input pandas DataFrame contains an additional column with datetime objects, `pygmt.grdtrack` mixes up the column names of the output pandas Dataframe.

Maybe this is partly related to the issue #2463?

### Minimal Complete Verifiable Example

```python
import pygmt
import pandas as pd
import numpy as np
import datetime

np.random.seed(100)

# Make random test data
df = pd.DataFrame(columns=["lon", "lat", "trash", "trashdate"])
df["lon"] = 50 + 2 * np.random.randn(100)
df["lat"] = 55 + 4 * np.random.randn(100)
df["trash"] = 53 + 5 * np.random.randn(100)
# Correct column names after applying grdtrack
# df["trashdate"] = 51 + 7 * np.random.randn(100)
# Mixed up column names after applying grdtrack
df["trashdate"] = (51 + 7 * np.random.randn(100)) \
* datetime.timedelta(seconds=3e7) + datetime.date(1950, 1, 1)

# Make mesh
spacing = 0.5
region = [min(df.lon), max(df.lon), min(df.lat), max(df.lat)]
n_lon = round((region[1] - region[0]) / spacing) + 1
n_lat = round((region[3] - region[2]) / spacing) + 1
lat, lon = np.mgrid[region[2]:region[3]:n_lat*1j, region[0]:region[1]:n_lon*1j]

# Make GMT ready grid with random z values
pygmt.xyz2grd(
x=lon.flatten(),
y=lat.flatten(),
z=50 + 1 * np.random.randn(len(lon.flatten())),
region=region,
spacing=spacing,
outgrid="test.grd",
)

# Extrakt the z values of the grid at the points stored in the pandas.DataFrame
# and write them to the new column "z_grid"
df_new = pygmt.grdtrack(
grid="test.grd",
points=df,
newcolname="z_grid",
)
```

Compare the pandas DataFrames:
```python
df
lon lat trash trashdate
0 46.500469 48.181395 56.022118 1994-11-01
1 50.685361 50.454956 48.464848 1998-02-18
2 52.306072 43.106738 55.960116 2019-08-08
3 49.495128 55.133269 50.814678 2020-09-02
4 51.962642 54.004445 53.508879 2009-02-25
.. ... ... ... ...
95 50.006035 57.596692 48.621346 1999-09-30
96 49.847953 54.300874 53.588042 1984-11-06
97 50.007915 59.069057 48.209158 2003-09-13
98 49.629972 52.600068 43.594427 1996-12-07
99 45.025697 61.304667 53.911714 1990-08-08

[100 rows x 4 columns]
```
and
```python
df_new
lon lat trash trashdate z_grid
0 46.500469 48.181395 56.022118 50.241717 1994-11-01
1 50.685361 50.454956 48.464848 50.100797 1998-02-18
2 52.306072 43.106738 55.960116 50.701331 2019-08-08
3 49.495128 55.133269 50.814678 48.872544 2020-09-02
4 51.962642 54.004445 53.508879 50.619094 2009-02-25
.. ... ... ... ... ...
95 50.006035 57.596692 48.621346 50.866452 1999-09-30
96 49.847953 54.300874 53.588042 50.468180 1984-11-06
97 50.007915 59.069057 48.209158 49.848821 2003-09-13
98 49.629972 52.600068 43.594427 50.984231 1996-12-07
99 45.025697 61.304667 53.911714 50.887035 1990-08-08

[100 rows x 5 columns]
```

### Full error message

No error message occurs, but the column names after applying `grdtrack` are mixed up in case the pandas DataFrame contains a column with datetime objects.

### System information

```bash
PyGMT information:
version: v0.10.1.dev25
System information:
python: 3.11.4 | packaged by conda-forge | (main, Jun 10 2023, 17:59:51) [MSC v.1935 64 bit (AMD64)]
executable: C:\ProgramData\Anaconda3\envs\pygmt_env_dev\python.exe
machine: Windows-10-10.0.19045-SP0
Dependency information:
numpy: 1.24.3
pandas: 2.0.2
xarray: 2023.1.1.dev17
netCDF4: 1.6.2
packaging: 23.1
contextily: 1.3.0
geopandas: 0.13.2
IPython: 8.14.0
rioxarray: 0.14.1
ghostscript: 9.54.0
GMT library information:
binary version: 6.4.0
cores: 4
grid layout: rows
image layout:
library path: C:/ProgramData/Anaconda3/envs/pygmt_env_dev/Library/bin/gmt.dll
padding: 2
plugin dir: C:/ProgramData/Anaconda3/envs/pygmt_env_dev/Library/bin/gmt_plugins
share dir: C:/Program Files (x86)/gmt6/share
version: 6.4.0
```