Do the flipping trick correctly. 

Now the entries of a square differentiation matrix are symmetric about the anti-diagonal with an opposite sign. The following cases are now exactly symmetric.

```
>> D = diffmat(3); D + D(end:-1:1,end:-1:1)
ans =
     0     0     0
     0     0     0
     0     0     0
>> D = diffmat(3, 'chebkind1'); D + D(end:-1:1,end:-1:1)
ans =
     0     0     0
     0     0     0
     0     0     0
>> D = diffmat(3, 'leg'); D + D(end:-1:1,end:-1:1)
ans =
     0     0     0
     0     0     0
     0     0     0
>> D = diffmat(4); D + D(end:-1:1,end:-1:1)
ans =
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
>> D = diffmat(4, 'chebkind1'); D + D(end:-1:1,end:-1:1)
ans =
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
>> D = diffmat(4, 'leg'); D + D(end:-1:1,end:-1:1)
ans =
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
```
This addresses #1149 and all tests pass.