pd.read_excel() ignores converter functions over na_filter argument
Please consider the code below.

```python
import pandas as pd
__to_str = lambda x: '' if pd.isnull(x) == True else str(x)

# Let's say that the excel to be read has a column with some empty strings or nulls in them
# by default na_filter is True
df = pd.read_excel('./file1.xlsx', converters = {'Name': __to_str}) # -> Leads to empty strings being read as nan

# when na_filter is False
df = pd.read_excel('./file1.xlsx', converters = {'Name': __to_str}, na_filter=False) # -> Leads to empty strings be read as empty strings (according to __to_str function) 

```
#### Why this behaviour?

I believe that if there's an explicitly provided argument(`converters`), its behaviour should override any other argument that has not been set(has a default value -> `na_filter`), **in case of such conflict or overlap of effect arises**. 

#### Expected output

When one argument is specified which operates at a certain specificity level, the other arguments having default values should give in to the effect caused by the explicitly set one.
For instance, if `converters` is provided with a function to handle cell values of a certain column, then `na_filter` or `keep_default_na` values which are applied by default and have not been passed explicitly.



Please correct me if my understanding is incorrect.

#### Output of `pd.show_versions()`
<details>
INSTALLED VERSIONS
------------------

commit           : None

pandas           : 1.0.0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.1.0
Cython           : 0.29.14
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : 2.7.7 (dt dec pq3 ext lo64)
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.13
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : 1.2.7
numba            : None
</details>
