Crash when using align between a Series and a DataFrame and ffill (or bfill) method
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
import pandas as pd

x = pd.Series(range(10), index=range(10))
y = pd.DataFrame(0.0, index=range(20), columns=range(12))

# works fine (no fill method)
x.align(y)

# crash
x.align(y, method='ffill')

# works fine if x is converted to a DataFrame
x.to_frame().align(y, method='ffill')



```
#### Problem description

This problem only occurs on pandas version >= 1.0.0
Pretty simple bug to reproduce... Quite a serious issue! It should probably shows elsewhere also.

**When using align between a Series and a DataFrame with a fill method the code hits this 

assert isinstance(obj, type(self)), type(obj)
AssertionError: <class 'pandas.core.frame.DataFrame'>**

My guess is that isinstance(obj, type(self)) try to assert that a Series is a DataFrame which is not the case.



#### Expected Output
Similar to x.align(y), with forward filled values.

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

<details>


INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 85 Stepping 4, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None
pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.0.3
setuptools       : 40.8.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : None
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : 0.48.0
</details>
