to_json() not including all attributes
`edit: simplified example`
#### Code Sample

```python
class MyClass:
    def __init__(self,value):
        self.value=value
        self.one=value+1
        self.two=value+2
        self.three=value+3
        
    def __repr__(self):
        return str(self.value)
    
# Sample 
x = MyClass(1)
x.__dict__
# Output
# {'value': 1, 'one': 2, 'two': 3, 'three': 4}

# Sample frame
values = [MyClass(value=x) for x in range(3)]
df = pd.DataFrame(values,columns=['amount'])

df.to_json()

```
#### Problem description

I have a custom class i'm using in dataframes, a simplified version of which is shown above. 

The output `to_json()` does not provide the full list of attributes, which I'd expected. Adding additional attributes to the class, *some* of them are included, but not all. 

#### Current output
```json
{"amount": {
    "0": {
        "one": 1,
        "two": 2
    },
    "1": {
        "one": 2,
        "two": 3
    },
    "2": {
        "one": 3,
        "two": 4
    }
}}
```
#### Expected Output
```json
{"amount":{
    "0": {
        "value": 0,
        "one": 1,
        "two": 2,
        "three": 3
    },
    "1": {
        "value": 1,
        "one": 2,
        "two": 3,
        "three": 4
    },
    "2": {
        "value": 2,
        "one": 3,
        "two": 4,
        "three": 5
    }
}
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.3.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0
Cython           : None
pytest           : 5.3.5
hypothesis       : None
sphinx           : 2.4.4
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.10.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.2
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.11
tables           : 3.6.1
tabulate         : 0.8.6
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : None
numba            : None
</details>
