Fix a bug when evaluating a periodic boundary condition for nonlinear ODEs.

@Hadrien-Montanelli and I just investigated this matter, and it turns out that when using the
```
N.bc = 'periodic';
```
the RHS of the BCs is set to be exactly, while when using the other syntax (lines 91-92) above,  one can enter values slightly off-zero (demonstrated for example via:
```
>> f = chebfun(@(x) cos(2*x),[-pi pi]);
>> fp = diff(f);
>> fp(pi)-fp(-pi)
ans =
   7.8160e-14
```

So this issue shouldn't be a cause for a particular concern, and I'll go ahead and make the merge!