varbinary types fail w/ mssql + pyodbc + NULL

[Mike Bayer]

inserting a NULL into a VARBINARY requires that pyodbc.BinaryNull be used. mxodbc probably has something similar. The tests have been failing for a long time for this type since we added VARBINARY as a default type for LargeBinary in #3039. which makes this a regression since 0.9 isn't doing that.

[Mike Bayer]

this also affects mxodbc. pymssql appears to be unaffected.

[Mike Bayer]

• Fixed issue when using :class:.VARBINARY type in conjunction with an INSERT of NULL + pyodbc; pyodbc requires a special object be passed in order to persist NULL. As the :class:.VARBINARY type is now usually the default for :class:.LargeBinary due to 3039, this issue is partially a regression in 1.0. The pymssql driver appears to be unaffected. fixes #3464

→ <<cset 78095940a437>>

[Mike Bayer]

still waiting on some contributors for mxodbc information