Do not deduce factories as references

Add a test proving that @ericniebler's suggestion to deduce `function`'s
factory argument as a value is necessary to accept lvalue factories, and
then take the suggestion to make the test pass.

Author: ispeters

include/exec/function.hpp

@@ -194,7 +194,7 @@ namespace experimental::execution
               && STDEXEC::__std::constructible_from<Factory>    //
               && STDEXEC::__callable<Factory, Args...>
               && STDEXEC::sender_to<STDEXEC::__invoke_result_t<Factory, Args...>, _receiver_t>
-      constexpr explicit _func_impl(Args &&...args, Factory &&factory)
+      constexpr explicit _func_impl(Args &&...args, Factory factory)
         noexcept(STDEXEC::__nothrow_move_constructible<Args...>)
         : args_(std::forward<Args>(args)...)
       {

test/exec/test_function.cpp

@@ -171,4 +171,13 @@ namespace
 
     REQUIRE(ret == 42);
   }
+
+  TEST_CASE("exec::function accepts lvalue callables", "[types][function]")
+  {
+    exec::function<int(int) noexcept> sndr(42, ex::just);
+
+    auto [ret] = ex::sync_wait(sndr).value();
+
+    REQUIRE(ret == 42);
+  }
 }  // namespace