resample() / DatetimeIndexResampler is not resampling according to the given frequency rule
When using frequency rule `2Q-SEP` it depends on the index of the time series it gives different resampled results. We should expect always March and September dates when using 2Q-SEP, however we might get June and December sometimes.
#### Code Sample, a copy-pastable example if possible

``` python
import pandas as pd

values = [100, 100, 100, 200, 200, 200, 200, 200, 200, 300, 300, 300]
index = pd.date_range('2016-01-01', '2016-12-31', freq='MS')

df = pd.DataFrame({'values': values}, index=index)
df_2Q_SEP = df.resample('2Q-SEP').mean()

# After resampling in 2Q-SEP we have March and September dates
print 'GOOD:'
print df_2Q_SEP
print

# Starting the TimeSeries in April
df_2 = pd.DataFrame({'values': values[3:]}, index=index[3:])
df_2_2Q_SEP = df_2.resample('2Q-SEP').mean()

# However here now after resampling in 2Q-SEP we have June and December dates
# which is not what we expected (given the desired frequency rule)
print 'BAD:'
print df_2_2Q_SEP
```
#### Expected Output

```
GOOD:                           
            values              
2016-03-31     100              
2016-09-30     200              
2017-03-31     300              

BAD:                            
            values              
2016-06-30     200              
2016-12-31     250                                              
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS                                            
------------------                                            
commit: None                                                  
python: 2.7.11.final.0                                        
python-bits: 64                                               
OS: Windows                                                   
OS-release: 7                                                 
machine: AMD64                                                
processor: Intel64 Family 6 Model 44 Stepping 2, GenuineIntel 
byteorder: little                                             
LC_ALL: None                                                  
LANG: None                                                    

pandas: 0.18.0                                                
nose: None                                                    
pip: 8.1.1                                                    
setuptools: 20.6.7                                            
Cython: None                                                  
numpy: 1.11.0                                                 
scipy: None                                                   
statsmodels: None                                             
xarray: None                                                  
IPython: None                                                 
sphinx: None                                                  
patsy: None                                                   
dateutil: 2.5.2                                               
pytz: 2016.3                                                  
blosc: None                                                   
bottleneck: None                                              
tables: None                                                  
numexpr: None                                                 
matplotlib: None                                              
openpyxl: None                                                
xlrd: None                                                    
xlwt: None                                                    
xlsxwriter: None                                              
lxml: None                                                    
bs4: None                                                     
html5lib: None                                                
httplib2: None                                                
apiclient: None                                               
sqlalchemy: None                                              
pymysql: None                                                 
psycopg2: None                                                
jinja2: None                                                  
boto: None                                                    
```
