Document default value for 'want_response_signed'

[jerrykan]

Document the default value for 'want_response_signed' so users don't have to dig through the code to fine it.

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[codecov]

Codecov Report

Merging #674 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #674   +/-   ##
=======================================
  Coverage   65.30%   65.30%           
=======================================
  Files         102      102           
  Lines       25568    25568           
=======================================
  Hits        16698    16698           
  Misses       8870     8870           

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[c00kiemon5ter]

thanks!