Join 2 data-frames on a common column (values mismatch)
i'm using pandas to read and join the 2 dataframe, but the data are not aligned on the column i chose for  the join. [Here](http://nbviewer.ipython.org/gist/epifanio/9126057) is reproducible example.

_Note at index 8 and 16, the values of SVSPP['_left'] and SVSPP do not match.
with my real data the mismatch is true for all the values and is always equal to 1._

You can see this issue clearly with the much smaller sub frame below:

```
df1 = pd.DataFrame({'SVSPP': {13: 14, 14: 15, 15: 16}, 'COMNAME': {13: 'CHAIN DOGFISH', 14: 'SPINY DOGFISH', 15: 'ATLANTIC ANGEL SHARK'}, 'SCINAME': {13: 'SCYLIORHINUS RETIFER', 14: 'SQUALUS ACANTHIAS', 15: 'SQUATINA DUMERIL'}})
df2 = pd.DataFrame({'ABUNDANCE': {8: 2, 7: 228}, 'CRUISE6': {8: 201105, 7: 201105}, 'SEASON': {8: 'FALL', 7: 'FALL'}, 'SVSPP': {8: 15, 7: 4}, 'BIOMASS': {8: 0.26200000000000001, 7: 5.7850000000000001}, 'LONGITUDE': {8: -74.630740000000003, 7: -74.630740000000003}, 'STATION': {8: 6, 7: 6}, 'LATITUDE': {8: 36.977443000000001, 7: 36.977443000000001}})

In [13]: df2.join(df1, on='SVSPP', lsuffix='_left')
Out[13]:
  SEASON  CRUISE6  STATION  SVSPP_left   LATITUDE  LONGITUDE  ABUNDANCE  BIOMASS  SVSPP               COMNAME           SCINAME
7   FALL   201105        6           4  36.977443  -74.63074        228    5.785    NaN                   NaN               NaN
8   FALL   201105        6          15  36.977443  -74.63074          2    0.262     16  ATLANTIC ANGEL SHARK  SQUATINA DUMERIL

In [14]: df2.merge(df1, on='SVSPP')
Out[14]:
  SEASON  CRUISE6  STATION  SVSPP   LATITUDE  LONGITUDE  ABUNDANCE  BIOMASS        COMNAME            SCINAME
0   FALL   201105        6     15  36.977443  -74.63074          2    0.262  SPINY DOGFISH  SQUALUS ACANTHIAS
```

See how SPINY DOGFISH appears in the second whilst ATLANTIC ANGEL SHARK is incorrectly in the first.

```
In [15]: df2.merge(df1, on='SVSPP')['COMNAME']
Out[15]:
0    SPINY DOGFISH
Name: COMNAME, dtype: object

In [16]: df2.join(df1, on='SVSPP', lsuffix='_left')['COMNAME']
Out[16]:
7                     NaN
8    ATLANTIC ANGEL SHARK
Name: COMNAME, dtype: object

In [17]: df2.merge(df1, on='SVSPP', how='left')['COMNAME']
Out[17]:
0              NaN
1    SPINY DOGFISH
Name: COMNAME, dtype: object
```
