0.12: join with on= fails with TimeStamp and Int64 MultiIndex
I encountered a very strange problem in 0.12 when a straightforward join fails while it works just fine in 0.11.

```
df1
```

> <class 'pandas.core.frame.DataFrame'>
> Int64Index: 71958 entries, 0 to 104014
> Data columns (total 3 columns):
> FactorDate         71958  non-null values
> FundSecNo          71958  non-null values
> SourceFundSecNo    71958  non-null values
> dtypes: datetime64[ns](1), int64(2)

```
df1.dtypes
```

> FactorDate         datetime64[ns]
> FundSecNo                   int64
> SourceFundSecNo             int64
> dtype: object

```
df2
```

> <class 'pandas.core.frame.DataFrame'>
> MultiIndex: 94868 entries, (2011-10-18 00:00:00, 283) to (2011-10-19 00:00:00, 127419)
> Data columns (total 2 columns):
> DemeanGroupId           94868  non-null values
> NormalizationGroupId    94868  non-null values
> dtypes: float64(1), int64(1)

```
for ind in df2.index.levels:
    print(type(ind))
```

> <class 'pandas.tseries.index.DatetimeIndex'>
> <class 'pandas.core.index.Int64Index'>

```
test = df1.join(df2, on=['FactorDate', 'SourceFundSecNo'])
test
```

> <class 'pandas.core.frame.DataFrame'>
> Int64Index: 71958 entries, 0 to 104014
> Data columns (total 5 columns):
> FactorDate              71958  non-null values
> FundSecNo               71958  non-null values
> SourceFundSecNo         71958  non-null values
> DemeanGroupId           0  non-null values
> NormalizationGroupId    0  non-null values
> dtypes: datetime64[ns](1), float64(2), int64(2)

```
date = datetime.datetime(2011, 10, 18)
sfsn = 283
test.loc[(test.FactorDate == date) & (test.SourceFundSecNo == sfsn)]
```

> ```
>           FactorDate  FundSecNo  SourceFundSecNo  DemeanGroupId  NormalizationGroupId
> ```
> 
> 1     2011-10-18 00:00:00        283              283            NaN                   NaN
> 17933 2011-10-18 00:00:00      65213              283            NaN                   NaN

```
df2.loc[(date, sfsn)]
```

> DemeanGroupId           350171
> NormalizationGroupId        35
> Name: (2011-10-18 00:00:00, 283), dtype: float64

I can attach the frames, but I forgot how to do it :-)
