Updated tests for itermonomials to support generator objects, using `next()` and `set()`. Also, added `raises()` for new `ValueError`'s in case of negative `max_degrees`.
Updated tests for itermonomials to support generator
objects, using `next()` and `set()`.
Also, added `raises()` for new `ValueError`'s in case of negative
`max_degrees`.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy