Fix multiple genre processing.

This isn't really a fix for ampache but for getid3 and I'll try to bring it upstream.

To me, the multiple genre processing is wrong because it tries to merge genre arrays together (using function array_merge_noclobber) on the basis of their index, not their content.

For example:
```
   [0] -> 'Trance'
```
merged with 
```
   [0] -> 'Rock'
```
results in
```
   [0] -> 'Trance'
```
because the index is the same. But really, the content should be compared and added if not in the array yet, like this:
```
   [0] -> 'Trance',
   [1] -> 'Rock'
```
So I added a new merge function that should do that merge properly.

This in turn makes the workaround for multiple genres in ampache redundant.