Adding dates as strings to url introduces extra characters

Hello,
Thanks for the great work that went into putting this package together. I just came across this package and have started using it. I ran into an issue and was hoping you can quickly clarify if I am missing something or if it is a bug ?
I am using python 3.7.5 and furl 2.1.0 and getting this weird behavior. Can you kindly help ?

```python
my_param_arg = dict()
my_param_arg["start_date"] = '20200301T08:00-0000'

my_url = furl("http://www.test.com/")
my_url.add(my_param_arg).url
```

Expected Behavior:
'http://www.test.com/?start_date=20200301T08%3A00-0000&start_date=20200301T08

Actual Behavior:
'http://www.test.com/?start_date=20200301T08%3A00-0000&start_date=20200301T08%3A00-0000'

Can you please explain why the extra characters(%3A00-0000) are being added to the url ? I would appreciate if you can offer a workaround as well.