Make reference(_internal) the default return value policy for properties (#473)

* Make reference(_internal) the default return value policy for properties

Before this, all `def_property*` functions used `automatic` as their
default return value policy. This commit makes it so that:

 * Non-static properties use `reference_interal` by default, thus
   matching `def_readonly` and `def_readwrite`.

 * Static properties use `reference` by default, thus matching
   `def_readonly_static` and `def_readwrite_static`.

In case `cpp_function` is passed to any `def_property*`, its policy will
be used instead of any defaults. User-defined arguments in `extras`
still have top priority and will override both the default policies and
the ones from `cpp_function`.

Resolves #436.

* Almost always use return_value_policy::move for rvalues

For functions which return rvalues or rvalue references, the only viable
return value policies are `copy` and `move`. `reference(_internal)` and
`take_ownership` would take the address of a temporary which is always
an error.

This commit prevents possible user errors by overriding the bad rvalue
policies with `move`. Besides `move`, only `copy` is allowed, and only
if it's explicitly selected by the user.

This is also a necessary safety feature to support the new default
return value policies for properties: `reference(_internal)`.

tests/test_methods_and_attributes.py

@@ -85,6 +85,44 @@ def test_static_properties():
     assert Type.def_property_static == 3
 
 
+@pytest.mark.parametrize("access", ["ro", "rw", "static_ro", "static_rw"])
+def test_property_return_value_policies(access):
+    from pybind11_tests import TestPropRVP
+
+    if not access.startswith("static"):
+        obj = TestPropRVP()
+    else:
+        obj = TestPropRVP
+
+    ref = getattr(obj, access + "_ref")
+    assert ref.value == 1
+    ref.value = 2
+    assert getattr(obj, access + "_ref").value == 2
+    ref.value = 1  # restore original value for static properties
+
+    copy = getattr(obj, access + "_copy")
+    assert copy.value == 1
+    copy.value = 2
+    assert getattr(obj, access + "_copy").value == 1
+
+    copy = getattr(obj, access + "_func")
+    assert copy.value == 1
+    copy.value = 2
+    assert getattr(obj, access + "_func").value == 1
+
+
+def test_property_rvalue_policy():
+    """When returning an rvalue, the return value policy is automatically changed from
+       `reference(_internal)` to `move`. The following would not work otherwise."""
+    from pybind11_tests import TestPropRVP
+
+    instance = TestPropRVP()
+    o = instance.rvalue
+    assert o.value == 1
+    o = TestPropRVP.static_rvalue
+    assert o.value == 1
+
+
 def test_dynamic_attributes():
     from pybind11_tests import DynamicClass, CppDerivedDynamicClass