as_bqm(n) needs check for int

**Description**

(sorry copy&paste from a JN terminal is iff.)

**Steps To Reproduce**
Stack Overflow provides an excellent guide on [how to create a Minimal, Complete and Verifiable example.](https://stackoverflow.com/help/mcve)

**Expected Behavior**
An error message that n must be an int

**Environment**
- OS: Ubuntu
- Python version: 3.6.7

**Additional Context**
I had a comment in the PR here: https://github.com/dwavesystems/dimod/pull/603/files
under
```
elif isinstance(cls, (Sequence, Set)):
```
Maybe I'll just add the check when I go in later