RVA2 tag can be found multiple times in ID3 v2.4

[jmudry]

Hello

According to the ID3 specifications (see section 4.11 at http://id3.org/id3v2.4.0-frames ), the RVA2 tag can be found multiple times in a file. Currently, parsing a file having multiple instances of this tag issues many warning messages.

Simply adding the RVA2 tag to the map multipleFrames removes the warning message and probably makes the tags being parsed more correctly (I haven't checked this point).

I've attached a patch file to the current master.

[IJabz]

Thanks.