[PROPOSAL] Make Series.update() accept a dictionary
#### Code Sample, a copy-pastable example if possible

I propose to re-write the `Series.update() ` method to accept a dictionary object. This would be useful for example when you are iterating over a DataFrame using `DataFrame.iterrows()`, and you want to update the `row` object returned by `iterrows()` in order to build another DataFrame starting from the DataFrame you are iterating over

Here is an example that doesn't work:

```python
import pandas as pd
s = pd.Series({'city':'Rome'})
s.update({'country':'Italy'})
```
#### Problem description

> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "C:\InstalledPrograms\Anaconda3\lib\site-packages\pandas\core\series.py", line 2807, in update
>     other = other.reindex_like(self)
> AttributeError: 'dict' object has no attribute 'reindex_like'

#### Simple use case

In this use case, an explicit conversion to dict() is needed

```
import random
import pandas as pd

def very_long_dictionary():
    d = dict()
    for c in range(2,10):
        d[c] = random.randint(1,101)
    return d

df = pd.DataFrame([[1,2],[3,4],[5,6]])
print(df)

outdl = list()
for index,row in df.iterrows():
    #Explicit conversion to dict is needed
    row = dict(row)
    row.update(very_long_dictionary())
    outdl.append(row)
newdf = pd.DataFrame(outdl)
print(newdf)
```
#### Expected Output

The series would update to the new value

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

<details>

>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.7.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.0.3
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3.post20200330
Cython           : 0.29.15
pytest           : 5.4.1
hypothesis       : 5.5.4
sphinx           : 2.4.4
blosc            : None
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.2
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.4.1
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.15
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.8
numba            : 0.48.0

</details>
