Strange behavior on saving a dataframe containing = at the start of a cell value
```python
import pandas as pd

index = [0, 1, 2]
text = ['====== 2', '======= abd', 'abc']
df = pd.DataFrame({'index':index, 'text': text})
df.to_excel('temp.xlsx', index=False)
print(df.head())

loaded_df = pd.read_excel('temp.xlsx')
print(loaded_df.head())

```
#### Problem description
Excel sheets interpret an `=` sign at the start of the cell as a signal for writing equations. If a column contains text data where one of the values start with `=` then the values are corrupted after saving the dataframe to an excel sheet.

#### Expected Output
```
   index         text
0      0     ====== 2
1      1  ======= abd
2      2          abc
```

#### Output after loading the exported xlsx sheet
```
   index text
0      0    0
1      1    0
2      2  abc
```
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 059f9bff62d3bfe53eeece409b48b879abd1a3b1
python           : 3.6.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-88-generic
Version          : #88-Ubuntu SMP Tue Feb 11 20:11:34 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : C.UTF-8
LANG             : en_US.utf8
LOCALE           : en_US.UTF-8

pandas           : 1.1.0.dev0+793.g059f9bff6
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.0.0
Cython           : 0.29.15
pytest           : 5.4.1
hypothesis       : 5.6.0
sphinx           : 2.4.4
blosc            : 1.8.3
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      : 0.3.3
gcsfs            : None
matplotlib       : 3.2.0
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.1
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.48.0
</details>
