Using pop(key, None) will remove the item without the risk of a KeyError being thrown

I ran into a key error being thrown when the dict didn't contain 'score'.

Introduced here:
https://github.com/toastdriven/django-haystack/commit/7247f6bb0d762c0ddec1675ccbedca57419475ef

Using pop with a default value will remove the risk of this happening. It could be a configuration issue on my end, but regardless, it's a no-frills change that removes a possible exception that wasn't being caught.