You are making assumptions of others motives based on your knowledge and emotions regarding the word which is different from people in the UK 60 odd years ago.

"Nigger" was a reasonably common name for black dogs then. There are a few other notable examples on the wikipedia page: http://en.wikipedia.org/wiki/Nigger_%28dog%29

People in the UK were racist as fuck back in the 40s (not as much as, say, white US southerners, of course)

Ok, let me put it another way. Prove the dog was called "Nigger" because Guy Gibson was racist... and "He must be racist otherwise he wouldn't have called the dog that" does not count since that is circular reasoning.

Unless the dog's name came from racist motivations the name itself (and the word) loses racial overtones UNLESS the listener decides to add them again in which case it is the listener who is at fault for assigning motivations the speaker (namer) in this case, didn't have.