Add more tests

[jwodder]

• Different header name normalizers (identity, hyphens=underscores, titlecase?, etc.)
• add_additional():
  • Calling add_additional() multiple times (some times with allow=False)
  • add_additional(False, extra arguments ...)
  • add_additional when a header has a dest that's just a normalized form of one of its names
• Calling add_field()/add_additional() on a HeaderParser after a previous call raised an error
• Scanning & parsing Unicode
• Normalizer that returns a non-string
• Non-string keys in NormalizedDict with the default normalizer
• Equality of HeaderParser objects
• Passing scanner options to HeaderParser
• Scanning files not opened in universal newlines mode