Error when Language in COMM is empty

[Mabenan]

At the moment an Exception NoSuchMethod is thrown when COMM is defined but without the language that is defined.

we should move

id3/lib/id3.dart

Line 272 in bc44531

if (metaTags['COMM'][language] == null) {

below

id3/lib/id3.dart

Line 275 in bc44531

}

Then it is ensured that

id3/lib/id3.dart

Line 276 in bc44531

metaTags['COMM'][language][shortDescription] = text;

wont throw an exception

[Mabenan]

#25 Should fix it