Prefer _SP over SP for default tag map space attrs
Prefer _SP over SP for default tag map space attrs

If `_SP` is already in the tag map, use the mapping from `_SP` instead
of `SP` so that `SP` can be a valid non-space tag. (Chinese has a
non-space tag `SP` which was overriding the mapping of `_SP` to
`SPACE`.)

explosion/spaCy,explosion/spaCy,honnibal/spaCy,explosion/spaCy,spacy-io/spaCy,spacy-io/spaCy,explosion/spaCy,explosion/spaCy,honnibal/spaCy,spacy-io/spaCy,explosion/spaCy,spacy-io/spaCy,spacy-io/spaCy,spacy-io/spaCy,honnibal/spaCy,honnibal/spaCy