BUG: upcasting on reshaping ops
#### Code Sample, a copy-pastable example if possible

```
import pandas as pd
import numpy as np

pn1 = pd.Panel(np.array([1.0], dtype=np.float32, ndmin=3)) # panel of 1.0
pn2 = pd.Panel(np.array([np.nan], dtype=np.float32, ndmin=3)) # panel of nan

print(pd.concat([pn1, pn1]).values.dtype)
print(pd.concat([pn2, pn2]).values.dtype)
```
#### Current Output

```
float32
float64
```
#### Expected Output

```
float32
float32
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.0
setuptools: 20.2.2
Cython: 0.23.4
numpy: 1.11.0
scipy: 0.17.0
statsmodels: None
xarray: None
IPython: 4.0.1
sphinx: 1.3.1
patsy: None
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: 1.1.0dev
tables: 3.2.2
numexpr: 2.5.1
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: 0.9.2
apiclient: None
sqlalchemy: 1.0.12
pymysql: 0.6.7.None
psycopg2: 2.6.1 (dt dec pq3 ext)
jinja2: 2.8
boto: None
pandas_datareader: None
```
