Fix duplicate kwarg algorithm when newton fallback is selected (#708)

* fix issue #700: duplicate kwarg algorithm

* fix spacing

* provide default to dict pop

python/fbprophet/forecaster.py

@@ -1048,6 +1048,7 @@ class Prophet(object):
                 params = model.optimizing(
                     dat, init=stan_init, iter=1e4, **kwargs)
             except RuntimeError:
+                kwargs.pop('algorithm', None)
                 params = model.optimizing(
                     dat, init=stan_init, iter=1e4, algorithm='Newton',
                     **kwargs