Fix string conversion under Clang

Author: wancw

src/json.cc

@@ -442,7 +442,7 @@ json::object_t json::get() const
     }
 }
 
-json::operator const std::string() const
+json::operator std::string() const
 {
     return get<std::string>();
 }

src/json.h

@@ -175,7 +175,7 @@ class json
     T get() const;
 
     /// implicit conversion to string representation
-    operator const std::string() const;
+    operator std::string() const;
     /// implicit conversion to integer (only for numbers)
     operator int() const;
     /// implicit conversion to double (only for numbers)

test/json_unit.cc

@@ -773,6 +773,7 @@ TEST_CASE("string")
         CHECK_NOTHROW(auto v = j.get<json::array_t>());
         CHECK_THROWS_AS(auto v = j.get<json::object_t>(), std::logic_error);
         CHECK_NOTHROW(auto v = j.get<std::string>());
+        CHECK_NOTHROW(auto v = static_cast<std::string>(j));
         CHECK_THROWS_AS(auto v = j.get<bool>(), std::logic_error);
         CHECK_THROWS_AS(auto v = j.get<int>(), std::logic_error);
         CHECK_THROWS_AS(auto v = j.get<double>(), std::logic_error);