Merge pull request #4374 from jreback/GH4318

BUG: Fixed passing keep_default_na=False when na_values=None (GH4318)
BUG:  Fixed bug with values raising an error on a DataFrame with duplicate columns and mixed
      dtypes, surfaced in (GH4377)

BUG:  Fixed bug with duplicate columns and type conversion in read_json when
      orient='split' (GH4377)