BUG: boolean frames multiplied by floats have dtypes=object
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
(pd.DataFrame(True, list('ab'), list('cd')) * 1.0).dtypes
#  returns object

type(True * 1.0)
# returns float
```
#### Problem description
A boolean frame multiplied by a float should return a DataFrame of floats to be consistent with python scalar operations (e.g., bool * float -> float).  Instead it returns dtypes = `object`.

Prior to 0.21.0, I believe this was the case?

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-36-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.21.0
pytest: 3.2.5
pip: 9.0.1
setuptools: 37.0.0
Cython: 0.27.3
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: 0.10.0
IPython: 5.5.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: 2.6.4
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: None
xlwt: 1.3.0
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0b10
sqlalchemy: 1.1.15
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.8.1
s3fs: None
fastparquet: None
pandas_gbq: 0.2.0
pandas_datareader: None

</details>
