BUG: convert_dtypes changes BooleanDtype to Int64
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> df = pd.DataFrame(data=[["abc", 123, True]])
>>> print(df)
     0    1     2
0  abc  123  True
>>> print(df.dtypes)
0    object
1     int64
2      bool
dtype: object
>>> df = df.convert_dtypes()
>>> print(df)
     0    1     2
0  abc  123  True
>>> print(df.dtypes)
0     string
1      Int64
2    boolean
dtype: object
>>> df = df.convert_dtypes()
>>> print(df)
        0    1  2
0  b'abc'  123  1
>>> print(df.dtypes)
0    object
1     Int64
2     Int64
dtype: object

```
#### Problem description

Applying convert_dtypes() to a column with dtype `string` converts it to a column dtype 'object' (and the individual values from `str` type to `bytes` type).

Applying convert_dtypes() to a column with dtype `boolean` converts it to a column dtype 'Int64' (and the individual values from `bool` type to `int` type).

#### Expected Output

convert_dtypes() should keep StringDtype columns as StringDtype and BooleanDtype columns as BooleanDtype.

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

<details>


pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.1.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_Malaysia.1252

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.2.3
setuptools       : 41.2.0
Cython           : None
pytest           : 5.3.5
hypothesis       : None
sphinx           : None
blosc            : None

</details>