insert_many throws ValueError with list of collections.Mapping instances
Fix pushed. It's not necessary to check both Mapping and MutableMapping, since MutableMappings are Mappings.
Hello. I've been using peewee in my latest project and I'm enjoying it quite a bit. Recently, I came across a problem when trying to pass instances of collections.Mapping to `insert_many()`, namely, it throws an ValueError exception. Here's an [example](https://gist.github.com/Jovito/2adbc33d7d60ddc6b6ebadccb750e8d6). From what I gathered, it seems that the offending line is: `is_dict = isinstance(row, dict)` in `insert_many()`. Changing it to `is_mapping = isinstance(row, (Mapping, MutableMapping))` solves the problem and still works if `row` is a regular dictionary. 