dataframe to_sql is inserting raw text into oracle database instead of actual text when utf-8 text is present 

#### Problem description

When trying to insert data into oracle database through dataframe to_sql it is taking long time so i have made changes type(sqlalchemy types) of object to VARCHAR using below code


```python
dtyp = {c:types.VARCHAR(df[c].str.len().max())
    for c in df.columns[df.dtypes == 'object'].tolist()}

df.to_sql(name='TEST_TABLE', con=engine, if_exists='append', index=False, chunksize=1000, dtype=dtyp)
```
Column of the dataframe contains text as utf-8, eg data is b'This is oracle'

b'This is oracle' is actual data present in the dataframe but the data is inserted in the oracle database is '415041432053656D69636F6E647563746F7' (example values) not the actual text 'This is oracle'. Seems like 'b' present before the text is causing the issue when converting it to VARCHAR if i trim out 'b' using decode UTF-8 i am getting UnicodeEncodeError: 'ascii' codec can't encode character error while inserting into database

Seems like to_sql having issue when utf-8 text is inserted as VARCHAR using dtypes



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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-957.1.3.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.24.2


</details>
