BulkImport no longer supports Nullable types

[vecknishwaran]

Describe the bug
Since upgrading from v1.7.0 to v1.7.1, the bulkimport function no longer supports nullable types. I receive the following error when attempting to import:

bulkcopy: mssql: invalid type for float column: sql.NullFloat64{%!s(float64=0) %!s(bool=true)}

I believe this issue was introduced following the changes made in pull request #179. I am willing to create a PR to address and fix this issue if needed.

[shueybubbles]

thx for opening an issue! A PR would be great. I suppose it means we're missing this test case?

go-mssqldb/bulkcopy_test.go

Line 17 in ada30cb

func TestBulkcopy(t *testing.T) {