Return a protocol error for upper case chars in a header name
Return a protocol error for upper case chars in a header name

- Use the new `NGHTTP2_HUFF_UPPER_CASE_CHAR` flag to determine if an upper
  case char is present in a header name
- Modify `contains_invalid_field_name_char` to set a different error message
  depending on whether we encountered a soft error or not

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