Move the error value when throwing an exception in .value rval overloads

Fixes #61

tests/issues.cpp

@@ -126,4 +126,13 @@ TEST_CASE("Issue 49", "[issues.49]") {
   auto m = test(10)
     .and_then(test2);
 }
-#endif
+#endif
+
+tl::expected<int, std::unique_ptr<std::string>> func()
+{
+  return 1;
+}
+
+TEST_CASE("Issue 61", "[issues.61]") {
+  REQUIRE(func().value() == 1);
+}