Specify initializers for yytest, token_string using initializer-lists

o We can retain -Weffc++ and specify default initializers by using
  initializer lists.  The risks are low (of additional non-conformat
  compilers), because there is already one other such initialization
  used in the code-base.

src/json.hpp

@@ -2833,29 +2833,30 @@ scan_number_done:
 
   private:
     /// input adapter
-    detail::input_adapter_t ia = nullptr;
+    detail::input_adapter_t ia { nullptr };
 
     /// the current character
-    int current = std::char_traits<char>::eof();
+    int current { std::char_traits<char>::eof() };
 
     /// the number of characters read
-    std::size_t chars_read = 0;
+    std::size_t chars_read { 0 };
+
     /// raw input token string (for error messages)
-    std::vector<char> token_string = std::vector<char>();
+    std::vector<char> token_string { };
 
     /// buffer for variable-length tokens (numbers, strings)
-    std::string yytext = "";
+    std::string yytext { };
 
     /// a description of occurred lexer errors
-    const char* error_message = "";
+    const char* error_message { "" };
 
     // number values
-    number_integer_t value_integer = 0;
-    number_unsigned_t value_unsigned = 0;
-    number_float_t value_float = 0;
+    number_integer_t value_integer { 0 };
+    number_unsigned_t value_unsigned { 0 };
+    number_float_t value_float { 0 };
 
     /// the decimal point
-    const char decimal_point_char = '.';
+    const char decimal_point_char { '.' };
 };
 
 /*!