Add lvalue ref-qualified cpp_function constructors (#2213)

* added overload for l-value ref-qualified methods

* Added test.
Before, the code would have failed to build.

tests/test_methods_and_attributes.py

@@ -510,3 +510,14 @@ def test_custom_caster_destruction():
 
     # Make sure we still only have the original object (from ..._no_destroy()) alive:
     assert cstats.alive() == 1
+
+
+def test_ref_qualified():
+    """Tests that explicit lvalue ref-qualified methods can be called just like their
+    non ref-qualified counterparts."""
+
+    r = m.RefQualified()
+    assert r.value == 0
+    r.refQualified(17)
+    assert r.value == 17
+    assert r.constRefQualified(23) == 40