add to_json method for C arrays

If the array can be converted to basic_json::string_t, the overload
in this commit is not chosen.

test/src/unit-conversions.cpp

@@ -190,6 +190,20 @@ TEST_CASE("value conversion")
 #endif
         }
 
+        SECTION("built-in arrays")
+        {
+          const char str[] = "a string";
+          const int nbs[] = {0, 1, 2};
+
+          json j2 = nbs;
+          json j3 = str;
+
+          auto v = j2.get<std::vector<int>>();
+          auto s = j3.get<std::string>();
+          CHECK(std::equal(v.begin(), v.end(), std::begin(nbs)));
+          CHECK(s == str);
+        }
+
         SECTION("std::deque<json>")
         {
             std::deque<json> a = j.get<std::deque<json>>();