model_to_json raises Object of type 'int64' is not JSON serializable on Model object

After training a prophet model with the following code

```python
model = Prophet(
holidays=self._specialPeriods,
growth='logistic',
changepoint_prior_scale=metric.changepoint_prior_scale,
seasonality_prior_scale=metric.seasonality_prior_scale
)

# Select the training features needed for this metric
trainingData = clientWeeklyData[['created_on', metric.name, *metric.features]]

# Set metric params on data
trainingData.rename({'created_on':'ds', metric.name:'y'}, axis=1, inplace=True) # Prepare data in correct format for the prophet.
trainingData['cap'] = metric.cap
trainingData['floor'] = metric.floor

model.add_country_holidays(country_name='UK')
for parameter in metric.features:
model.add_regressor(parameter)

self._models.setdefault(client, {})[metric.name] = model.fit(trainingData)
```

where `trainingData` and `self._specialPeriods` are pandas data frames

```
# trainingData.dtypes
ds datetime64[ns]
y float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
**** float64
cap int64
floor int64
```
You receive the following error when you come to save the model using model_to_json

```
>>>model_to_json(model)
Traceback (most recent call last):
File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/home/vscode/.vscode-server/extensions/ms-python.python-2021.3.658691958/pythonFiles/lib/python/debugpy/__main__.py", line 45, in
cli.main()
File "/home/vscode/.vscode-server/extensions/ms-python.python-2021.3.658691958/pythonFiles/lib/python/debugpy/../debugpy/server/cli.py", line 444, in main
run()
File "/home/vscode/.vscode-server/extensions/ms-python.python-2021.3.658691958/pythonFiles/lib/python/debugpy/../debugpy/server/cli.py", line 331, in run_module
run_module_as_main(target_as_str, alter_argv=True)
File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/workspaces/optimised_package_margin/components/pipeline/pipeline/__main__.py", line 100, in
model.save(args.model_path)
File "/workspaces/optimised_package_margin/components/pipeline/pipeline/forecast_model.py", line 331, in save
json.dump(model_to_json(model), handle)
File "/home/vscode/.local/lib/python3.6/site-packages/fbprophet/serialize.py", line 91, in model_to_json
return json.dumps(model_json)
File "/usr/lib/python3.6/json/__init__.py", line 231, in dumps
return _default_encoder.encode(obj)
File "/usr/lib/python3.6/json/encoder.py", line 199, in encode
chunks = self.iterencode(o, _one_shot=True)
File "/usr/lib/python3.6/json/encoder.py", line 257, in iterencode
return _iterencode(o, 0)
File "/usr/lib/python3.6/json/encoder.py", line 180, in default
o.__class__.__name__)
TypeError: Object of type 'int64' is not JSON serializable
```
Versions:
Python 3.6.9
fbprophet 0.7.1
pandas 1.1.5

Please could you take a look into this problem - in the meantime, do I need to convert the DataFrame into something else before I pass it in to avoid having the np.int64?