UTF8 in TextField's 'default'

Like https://github.com/ho-tex/hyperref/issues/5, LuaLaTeX produces an incorrect result if you use non-ASCII characters for 'default' in TextField.
It seems pdfLaTeX produces the correct result if you use only LATIN1-characters, but this problem occurs on pdfLaTeX too, if you use Japanese characters for 'default'.
The following hack fixes this problem.
(dank des Kommentars von Frau Fischer in https://github.com/ho-tex/hyperref/issues/5)

\documentclass{article}

\usepackage{fontspec}		% ---- for LuaLaTeX
% \usepackage[utf8]{inputenc}	% ---- for pdfLaTeX

\usepackage[unicode=true]{hyperref}
\usepackage[T1]{fontenc}

% FIX for 'default'
% See also the fix for 'value' by Frau Fischer
% ( https://github.com/ho-tex/hyperref/issues/5 ).
% I just replaced 'value' with 'default'.
\makeatletter
\define@key{Field}{default}{%
  \Hy@pdfstringdef\Fld@default{#1}}
\makeatother

% NOTE:
%   This fix IS needed for pdfLaTeX too, when you use Japanese
%   characters.
%
% \documentclass{article}
% \usepackage[whole]{bxcjkjatype}
% \usepackage[utf8]{inputenc}
% \begin{document}
% \begin{Form}
%   % without fix, fails to compile
%   \TextField[name=addr,default=東京]{Address} %Tokyo
% \end{Form}
% \end{document}

\begin{document}
\begin{Form}
  % OK, of course
  \TextField[name=textfield1]{Address} \\

  % OK
  \TextField[name=textfield2,value=Köln]{Address} \\

  % correct on pdfLaTeX (but incorrect if you use Japanese characters)
  % incorrect without FIX on LuaLaTeX
  \TextField[name=textfield3,default=München]{Address} \\

  % (though this is probably meaningless)
  % incorrect (internally) on LuaLaTeX
  % 
  % $ pdftk pr-textfield-default-encoding.pdf dump_data_fields
  % ...
  % FieldValue: Köln			% This is OK, but
  % FieldValueDefault: München	% Quatsch!
  % ...
  \TextField[name=textfield4,value=Köln,default=München]{Address}
\end{Form}
\end{document}

Start by compiling the provided minimal examples with LuaLaTeX and pdfLaTeX, then compare the behavior with the workaround shown in the issue and the related hyperref issue #5. Done means non-ASCII TextField default values, including Japanese characters, are encoded correctly without the workaround.