Fix issue #3551: Upper case lemmas
Fix issue #3551: Upper case lemmas

If the Morphology class tries to lemmatize a word that's not in the
string store, it's forced to just return it as-is. While loading
exceptions, the class could hit a case where these strings weren't in
the string store yet. The resulting lemmas could then be cached, leading
to some words receiving upper-case lemmas. Closes #3551.

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