Test the C++11 stand-in for std::string_view of json_view

Six members of string_ref (length, begin, end, operator[], operator!=,
and operator<<) were not reached before C++17, where string_ref is
std::string_view.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view_builder.cpp

@@ -10,6 +10,7 @@
 
 #include <nlohmann/json.hpp>
 #include <nlohmann/detail/view/builder.hpp>
+#include <nlohmann/detail/view/string_ref.hpp>
 using nlohmann::json;
 
 #include <cstdint>
@@ -214,6 +215,22 @@ struct generator
 };
 } // namespace
 
+TEST_CASE("json_view string_ref")
+{
+    // std::string_view in C++17, a stand-in with the same members before
+    using nlohmann::detail::view::string_ref;
+    const std::string text = "abc";
+    const string_ref r(text);
+    CHECK(r.length() == 3);
+    CHECK(std::string(r.begin(), r.end()) == "abc");
+    CHECK(r[1] == 'b');
+    CHECK(r != string_ref("abd"));
+    CHECK_FALSE(r != string_ref("abcd", 3));
+    std::ostringstream o;
+    o << r;
+    CHECK(o.str() == "abc");
+}
+
 TEST_CASE("json_view builder")
 {
     SECTION("scalars and containers")