Corrected merging of multiple genres for ID3v2.

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 add new genres while increasing their index.