`NSMutableArray.arrayWithContentsOfFile` has different return types

[liamappelbe]

NSMutableArray? arrayWithContentsOfFile:(NSString path)
NSArray? arrayWithContentsOfFile:(NSString path)

There are a few cases like this and it's one remaining source of "duplicate method" log spam. The important thing is that NSMutableArray is a subtype of NSArray, so we probably need to add some sort of subtyping test to handle this case. The fix is just to detect this case and keep the subtype version.