Define column-wise na/default value for merge operations.
I find myself often using fillna on a lot of columns after a merge operation and having to cast columns back to their original dtype.
Pseudo code now 

data_types = df.dtypes
df_new = merge(df, df_other)
for col in former_int64_columns:
    df_new[col] = df_new[col].fillna(0).astype(np.int64)

in some cases I may want a different default value, also for other column types than int64.

An idea would be something like a keyword argument "column_default_values = { 'a': 0, 'b':  -1, ...}".
