Rename inspectdb tests model's field for clarity
|Reported by:||charettes||Owned by:||nobody|
|Has patch:||yes||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
The recently added inspect db test refers to model field by numerical index.
The attached patch name those fields based on their type for clarity. This also fixes the regression on MySQL concerning blank and null ordering in NullBooleanField.