DataFrame.to_excel with xlsxwriter and constant_memory makes most of the cells empty
#### Problem description

Hello,

When we export data frames to excel using `xlsxwriter` with the option `constant_memory` set to `True`, most of the cells are empty.

Thanks a lot

#### Code Sample

```python
import seaborn.apionly as sns
import pandas as pd
iris = sns.load_dataset('iris')
with pd.ExcelWriter('constant_memory_false.xlsx', engine='xlsxwriter', options=dict(constant_memory=False)) as xlw:
    iris.to_excel(xlw)  # Open it: this is OK
with pd.ExcelWriter('constant_memory_true.xlsx', engine='xlsxwriter', options=dict(constant_memory=True)) as xlw:
    iris.to_excel(xlw)  # Open it: most of the data are missing
```

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

<details>
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-87-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 34.1.1
Cython: 0.25.2
numpy: 1.12.0
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.2.2
sphinx: 1.5.2
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.2
matplotlib: 2.0.0
openpyxl: None
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: None
bs4: 4.5.3
html5lib: 0.9999999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.9.5
boto: None
pandas_datareader: None
</details>
