Pass a mutable string in unit test
Pass a mutable string in unit test

This resolves a clang diagnostics warning due to parse_http_message
accepting 'char *' as it's first argument

GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver,GerHobbelt/civet-webserver