Fix for trigcoeffs bug when even number coefficients requested

The `trigtech/trigcoeffs` method is not returning the correct coefficients when an even number is requested.  For example,
```
>> f = chebfun('1 + cos(pi*x)','trig');
>> trigcoeffs(f,2)
ans =
   1.0000 + 0.0000i
   0.5000 - 0.0000i
```
This also resulted in incorrect results when using the `trunc` flag:
```
>> f = chebfun('1 + cos(pi*x)','trig','trunc',2)
f =
   chebfun column (1 smooth piece)
       interval       length   endpoint values trig
[      -1,       1]        2      -0.5     -0.5 
Epslevel = 1.480297e-15.  Vscale = 1.500000e+00.
```
Note the end point values indicate the result is off.

This pull request fixes this bug and includes tests for even number trig coefficient requests.