next_prime() doesn't actually return the next prime (it can skip some)

[darconeous]

Not sure if this is intended behavior, but I noticed that next_prime() seems to skip over some primes; it doesn't always pick the next prime.

As an example, if I give it the number 1032989, the next prime is actually 1033001. However, next_prime() ends up returning 1033027.

Again, not sure if this is intended behavior, but if this is the intended behavior then it should likely be called out in the documentation for the method.

Maybe this is due to #44?

[darconeous]

Yep, I can confirm that fixing #44 appears to fix this problem.