Fix convolution none bug #1579

Here ist what I did:
1. I fixed the behaviour of convolve in `boundary=None` mode and added a few more tests.
2. I changed the default behaviour of `convolve` to `'fill'` so it is the same as `convolve_fft`.
3. I added further test for `convolve_fft` and fixed the masked array handling, which was 
buggy because the array was converted to a normal array, before storing the mask.

@astrofrog It would be great if you could check the changes for `boundary=None` in `convolve`.