Fix label-clobber bug in parser.begin_training()
Fix label-clobber bug in parser.begin_training()

The parser.begin_training() method was rewritten in v2.1. The rewrite
introduced a regression, where if you added labels prior to
begin_training(), these labels were discarded. This patch fixes that.

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