BUG: Inconsistent behaviour when averaging Decimals, floats and ints
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

---

```python
from decimal import Decimal

df = pd.DataFrame({'col_1': [Decimal(1.5), Decimal(4.0)], 'col_2': [5.0, 10.0]})
df.mean(axis=1) # returns 5, 10 -- ignoring the decimals types in the averaging

df2 = pd.DataFrame({'col_1': [Decimal(1.5), Decimal(4.0)], 'col_2': [5, 10]})
df2.mean(axis=1) # returns 3.25, 7 -- includes the decimals types in the averaging
```

#### Problem description

There is inconsistent behaviour on how Decimal is being averaging depending if it is averaged to an int vs a float. Is it expected that the two dataframes above return different results?

#### Expected Output

I would expect in both cases to see `3.25` and  `7` as the mean of the rows.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.18.3
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3
Cython           : None
pytest           : 5.4.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.5 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.4.1
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.16
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : 1.2.8
numba            : None

</details>
