Fix dropbox#38

Parse the number according to the JSON grammar, ignoring the current
locale.

json11.cpp

@@ -24,7 +24,9 @@
 #include <cmath>
 #include <cstdlib>
 #include <cstdio>
+#include <cstring>
 #include <limits>
+#include <sstream>
 
 namespace json11 {
 
@@ -321,6 +323,23 @@ bool Json::operator< (const Json &other) const {
  * Parsing
  */
 
+/* strtod_dot(str)
+ *
+ * Parse the number according to the JSON grammar, ignoring the current locale assuming that the
+ * string is already validated to comply with the JSON grammar.
+ */
+double strtod_dot(const char* str) {
+    // It is not correct to use ordinary strtod because it uses the locale and in some locales,
+    // for example, in ru_RU.UTF-8, the floating-point delimiter is "," but not "." as in JSON
+    // grammar.
+    const size_t str_length = strspn(str, "0123456789.eE+-");
+    std::istringstream string_stream(std::string(str, str_length));
+    string_stream.imbue(std::locale("C"));
+    double f = NAN;
+    string_stream >> f;
+    return f;
+}
+
 /* esc(c)
  *
  * Format char c suitable for printing in an error message.

test.cpp

@@ -252,6 +252,18 @@ JSON11_TEST_CASE(json11_test) {
 
 }
 
+JSON11_TEST_CASE(json11_test_numbers) {
+    const std::string numbers =
+        R"({"numbers": [0, -0, 1, 42, 0.5, -0.5, 0.5e+1, 0.5E+1, 5.0e-1, 0.5e+12]})";
+    const std::string expected_numbers =
+        R"({"numbers": [0, 0, 1, 42, 0.5, -0.5, 5, 5, 0.5, 500000000000]})";
+
+    std::string err;
+    const std::string numbers_json = Json::parse(numbers, err).dump();
+    std::cout << "numbers_json: " << numbers_json << std::endl;
+    JSON11_TEST_ASSERT(numbers_json == expected_numbers);
+}
+
 #if JSON11_TEST_STANDALONE_MAIN
 
 static void parse_from_stdin() {
@@ -277,6 +289,7 @@ int main(int argc, char **argv) {
     }
 
     json11_test();
+    json11_test_numbers();
 }
 
 #endif // JSON11_TEST_STANDALONE_MAIN