Series constructor ignores copy=True when dtype agrees
#### Code Sample

```python
>>> %paste
import pandas as pd

x = pd.Series([1.0])
y = pd.Series(x, copy=True)
z = pd.Series(x, copy=True, dtype=float)

x[0] = 2.0
print(x[0])
print(y[0])
print(z[0])
## -- End pasted text --
2.0
1.0
2.0
```

#### Problem description

Constructing a Series with copy=True should return a copy of the original, but when the dtype argument agrees with the source dtype, no copy is performed.

#### Expected Output

```python
2.0
1.0
1.0
```

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

<details>
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.1.13-100.fc21.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: C
LANG: C
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.1
numpy: 1.11.2
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 4.2.0
sphinx: 1.4.8
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: 1.1.0
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: 2.4.0
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.1.3
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.43.0
pandas_datareader: None
</details>
