BUG: update should try harder to preserve dtypes

more examples in #13957 

http://stackoverflow.com/questions/17398216/unwanted-type-conversion-in-pandas-dataframe-update

```
df = pd.DataFrame({'int': [1, 2], 'float': [np.nan, np.nan]})

print('Integer column:')
print(df['int'])

for _, df_sub in df.groupby('int'):
    df_sub['float'] = float(df_sub['int'])
    df.update(df_sub)

print('NO integer column:')
print(df['int']) 
```
