BUG:  fixed length string data type specifiers, are ignored on astype calls, if they are passed in a dictionary
- [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.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
def test_astype_string_conversion(): 
    
    df = pd.DataFrame([{'n': 0, 's':'qwerty'}] * 3)

    direct_conversion_df = df.copy()
    direct_conversion_df['s'] = direct_conversion_df['s'].astype('|S10')

    # direct column conversion : String to fixed len bytes works as expected
    assert direct_conversion_df['s'].dtype != np.dtype('O')

    # Where as type specified with a dictionary
    dictionary_conversion_df = df.astype({'n': 'float64', 's':'|S10'})    
    
    # the float conversion works, 
    assert dictionary_conversion_df['n'].dtype == np.dtype('float64')
    
    # but the strings are still objects, 
    assert dictionary_conversion_df['s'].dtype != np.dtype('O')  # FAIL

```

#### Problem description

pandas.DataFrame.astype,  'dtype' parameter accepts a dictionary which can specify the columns to be converted.  It looks like limited length string specifiers, e.g: '|S10', are ignored, if they are specified in the dictionary.  Directly converting columns to the same type, works as expected. 

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

<details>

INSTALLED VERSIONS
------------------
commit           : 67a3d4241ab84419856b84fc3ebc9abcbe66c6b3
python           : 3.8.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.104-microsoft-standard
Version          : #1 SMP Wed Feb 19 06:37:35 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.4
numpy            : 1.19.4
pytz             : 2020.4
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.3.1.post20200622
Cython           : 0.29.21
pytest           : 5.4.3
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : 1.1
pymysql          : None
psycopg2         : 2.8.5 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : 0.8.4
fastparquet      : 0.4.1
gcsfs            : None
matplotlib       : 3.2.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 2.0.0
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.0
sqlalchemy       : 1.3.18
tables           : 3.6.1
tabulate         : None
xarray           : 0.16.1
xlrd             : None
xlwt             : None
numba            : 0.51.2

</details>
