Update json_parser.cpp
Update json_parser.cpp

fix the narrowing conversion error with g++ on alpine:
/usr/src/nginx-openidc/src/json-api-core/json_parser.cpp: At global scope:
/usr/src/nginx-openidc/src/json-api-core/json_parser.cpp:106:81: error: narrowing conversion of '192' from 'int' to 'char' inside { } [-Wnarrowing]
 static const char firstByteMark[7] = { 0x00, 0x00, 0xC0, 0xE0, 0xF0, 0xF8, 0xFC };
                                                                                 ^
/usr/src/nginx-openidc/src/json-api-core/json_parser.cpp:106:81: error: narrowing conversion of '224' from 'int' to 'char' inside { } [-Wnarrowing]
/usr/src/nginx-openidc/src/json-api-core/json_parser.cpp:106:81: error: narrowing conversion of '240' from 'int' to 'char' inside { } [-Wnarrowing]
/usr/src/nginx-openidc/src/json-api-core/json_parser.cpp:106:81: error: narrowing conversion of '248' from 'int' to 'char' inside { } [-Wnarrowing]
/usr/src/nginx-openidc/src/json-api-core/json_parser.cpp:106:81: error: narrowing conversion of '252' from 'int' to 'char' inside { } [-Wnarrowing]
tarachandverma/ngx-openidc,tarachandverma/ngx-openidc,tarachandverma/ngx-openidc