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`.)

spacy/morphology.pyx

@@ -152,6 +152,9 @@ cdef class Morphology:
         self.tags = PreshMap()
         # Add special space symbol. We prefix with underscore, to make sure it
         # always sorts to the end.
+        if '_SP' in tag_map:
+            space_attrs = tag_map.get('_SP')
+        else:
             space_attrs = tag_map.get('SP', {POS: SPACE})
         if '_SP' not in tag_map:
             self.strings.add('_SP')