Series constructor skips dtype=str conversion for list data
#### Code Example

```python
from pandas import Series, DataFrame

int_list = [1, 2, 3]

s1 = Series(int_list)
s2 = Series(int_list, dtype=float)
s3 = Series(int_list, dtype=str)
s4 = Series(int_list, dtype='U')
s5 = Series(Series(int_list), dtype=str)

print('Series element type:')
print('  s1:', type(s1[0]))
print('  s2:', type(s2[0]))
print('  s3:', type(s3[0]))
print('  s4:', type(s4[0]))
print('  s5:', type(s5[0]))

f1 = DataFrame(int_list)
f2 = DataFrame(int_list, dtype=float)
f3 = DataFrame(int_list, dtype=str)
f4 = DataFrame(int_list, dtype='U')
f5 = DataFrame(DataFrame(int_list), dtype=str)

print('\nDataFrame element type:')
print('  f1:', type(f1.iloc[0, 0]))
print('  f2:', type(f2.iloc[0, 0]))
print('  f3:', type(f3.iloc[0, 0]))
print('  f4:', type(f4.iloc[0, 0]))
print('  f5:', type(f5.iloc[0, 0]))
```
Output:
```
Series element type:
  s1: <class 'numpy.int64'>
  s2: <class 'numpy.float64'>
  s3: <class 'int'>
  s4: <class 'int'>
  s5: <class 'str'>

DataFrame element type:
  f1: <class 'numpy.int64'>
  f2: <class 'numpy.float64'>
  f3: <class 'str'>
  f4: <class 'str'>
  f5: <class 'str'>
```

#### Problem description

When creating a `Series` from a `list` using `dtype=str`, the data elements are not converted to strings. The `Series` instance apparently just keeps the original (Python) data type in this case.

This problem does not occur when, instead of a `list`, another `Series` is used as input data (`s5` in the example above). It also does not happen when creating `DataFrame` instances from `list` data.

#### Expected Output

```
Series element type:
  s1: <class 'numpy.int64'>
  s2: <class 'numpy.float64'>
  s3: <class 'str'>
  s4: <class 'str'>
  s5: <class 'str'>

DataFrame element type:
  f1: <class 'numpy.int64'>
  f2: <class 'numpy.float64'>
  f3: <class 'str'>
  f4: <class 'str'>
  f5: <class 'str'>
```

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

<details>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.11.3-1-ARCH
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.1
pytest: 3.1.1
pip: 9.0.1
setuptools: 36.0.1
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 5.3.0
sphinx: 1.6.1
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: 1.1.10
pymysql: None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: 0.4.0
```

</details>
