[MSSQL] Identity insert not working

[dirkjonker]

To be able to insert a value in an identity column (like an auto-increment column) you need to explicitly set IDENTITY_INSERT on that specific table to ON. Since turbodbc closes the statement handle after execute, this setting is not remembered, and a second execute that tries to insert will fail.

Related to #90

cur.execute("SET IDENTITY_INSERT ##test_some ON")
Out[38]: <turbodbc.cursor.Cursor at 0x111a0efd0>

In [39]: cur.execute("INSERT INTO ##test_some (val) VALUES (?)", (1,))
---------------------------------------------------------------------------
Error                                     Traceback (most recent call last)
/Users/Dirkjonker/.pyenv/versions/3.5.2/envs/turbo/lib/python3.5/site-packages/turbodbc/exceptions.py in wrapper(*args, **kwds)
     34         try:
---> 35             return f(*args, **kwds)
     36         except InternError as e:

/Users/Dirkjonker/.pyenv/versions/3.5.2/envs/turbo/lib/python3.5/site-packages/turbodbc/cursor.py in execute(self, sql, parameters)
     71             buffer.add_set(parameters)
---> 72             buffer.flush()
     73         self.impl.execute()

Error: ODBC error
state: 23000
native error code: 544
message: [Microsoft][ODBC Driver 13 for SQL Server][SQL Server]Cannot insert explicit value for identity column in table '##test_some' when IDENTITY_INSERT is set to OFF.

During handling of the above exception, another exception occurred:

DatabaseError                             Traceback (most recent call last)
<ipython-input-39-ca28f00caca6> in <module>()
----> 1 cur.execute("INSERT INTO ##test_some (val) VALUES (?)", (1,))

/Users/Dirkjonker/.pyenv/versions/3.5.2/envs/turbo/lib/python3.5/site-packages/turbodbc/exceptions.py in wrapper(*args, **kwds)
     35             return f(*args, **kwds)
     36         except InternError as e:
---> 37             raise DatabaseError(str(e))
     38     return wrapper

DatabaseError: ODBC error
state: 23000
native error code: 544
message: [Microsoft][ODBC Driver 13 for SQL Server][SQL Server]Cannot insert explicit value for identity column in table '##test_some' when IDENTITY_INSERT is set to OFF.