to_latex with float_format doesn't work like expected 
#### Code Sample

```python
import pandas as pd
df = pd.DataFrame([[0.19999]])
print(df.to_latex(float_format='%.3f'))
# returns:
# \begin{tabular}{lr}
# \toprule
# {} &   0 \\
# \midrule
# 0 & 0.2 \\
# \bottomrule
# \end{tabular}
```
#### Problem description

The float format doesn't seem to be respected in the example above. In contrast:
```python
'%.3f' % df.values[0][0]
# returns
# '0.200'
```
I would expect the number to be rendered as `0.200` instead of `0.2`.

Issue #6052 is possibly related, but the problem description is different.

#### Expected Output

```
\begin{tabular}{lr}
\toprule
{} &   0 \\
\midrule
0 & 0.200 \\
\bottomrule
\end{tabular}
```

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

<details>
Using Anaconda Python 3.6.5 with recent pandas 0.23.1.

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-128-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.1
pytest: None
pip: 10.0.1
setuptools: 39.2.0
Cython: None
numpy: 1.14.5
scipy: None
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

```
</details>
