BUG/SQL: to_sql  sqlalchemy.exc.DataError for datetime64[ns] column in Redshift
I found that when using `to_sql` on a dataframe with a datetime64 column with most of the values being `NaT`, sqlalchemy fails. The column `closed_at` is of type `TIMESTAMP` on redshift (Timestamp without timezone).

```
df.info()
[output]
<class 'pandas.core.frame.DataFrame'>
Int64Index: 11 entries, 0 to 1684
Data columns (total 11 columns):
created_at         11 non-null datetime64[ns]
updated_at         11 non-null datetime64[ns]
closed_at          2 non-null datetime64[ns]
state              11 non-null object
title              11 non-null object
body               11 non-null object
issue_submitter    11 non-null object
assignee           6 non-null object
labels             11 non-null object
html_url           11 non-null object
id                 11 non-null int64
dtypes: datetime64[ns](3), int64(1), object(7)None

```

The column `closed_at` has only 2 non empty values. if I delete the column prior to writing everything goes fine. However,  when i call `to_sql` with the column on the dataframe i get the following error:

```
Traceback (most recent call last):
  df.to_sql(table, self.engine,if_exists='append',index=False)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/pandas/core/generic.py", line 950, in to_sql
    index_label=index_label)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/pandas/io/sql.py", line 475, in to_sql
    index_label=index_label)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/pandas/io/sql.py", line 842, in to_sql
    table.insert()
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/pandas/io/sql.py", line 611, in insert
    self.pd_sql.execute(ins, data_list)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/pandas/io/sql.py", line 810, in execute
    return self.engine.execute(*args, **kwargs)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1682, in execute
    return connection.execute(statement, *multiparams, **params)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 720, in execute
    return meth(self, multiparams, params)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 317, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 817, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 947, in _execute_context
    context)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1108, in _handle_dbapi_exception
    exc_info
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 185, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 917, in _execute_context
    context)
  File "/home/manugarri/anaconda/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 432, in do_executemany
    cursor.executemany(statement, parameters)
sqlalchemy.exc.DataError: (DataError) invalid input syntax for type timestamp: "0001-255-255T00:00:00"
```
