Fix assertion failure for unions (pybind#1685)

In def_readonly and def_readwrite, there is an assertion that the member comes
from the class or a base class:

    static_assert(std::is_base_of<C, type>::value, "...");

However, if C and type are the same type, is_base_of will still only be true
if they are the same _non-union_ type.  This means we can't define accessors
for the members of a union type because of this assertion.

Update the assertion to test

    std::is_same<C, type>::value || std::is_base_of<C, type>::value

which will allow union types, or members of base classes.

Author: rolandd

include/pybind11/pybind11.h

@@ -1176,7 +1176,7 @@ class class_ : public detail::generic_type {
 
     template <typename C, typename D, typename... Extra>
     class_ &def_readwrite(const char *name, D C::*pm, const Extra&... extra) {
-        static_assert(std::is_base_of<C, type>::value, "def_readwrite() requires a class member (or base class member)");
+        static_assert(std::is_same<C, type>::value || std::is_base_of<C, type>::value, "def_readwrite() requires a class member (or base class member)");
         cpp_function fget([pm](const type &c) -> const D &{ return c.*pm; }, is_method(*this)),
                      fset([pm](type &c, const D &value) { c.*pm = value; }, is_method(*this));
         def_property(name, fget, fset, return_value_policy::reference_internal, extra...);
@@ -1185,7 +1185,7 @@ class class_ : public detail::generic_type {
 
     template <typename C, typename D, typename... Extra>
     class_ &def_readonly(const char *name, const D C::*pm, const Extra& ...extra) {
-        static_assert(std::is_base_of<C, type>::value, "def_readonly() requires a class member (or base class member)");
+        static_assert(std::is_same<C, type>::value || std::is_base_of<C, type>::value, "def_readonly() requires a class member (or base class member)");
         cpp_function fget([pm](const type &c) -> const D &{ return c.*pm; }, is_method(*this));
         def_property_readonly(name, fget, return_value_policy::reference_internal, extra...);
         return *this;