DnAttribute annotation ignores value if index is present

[marcusdacoregio]

The DefaultObjectDirectoryMapper ignores the value attribute from @DnAttribute if index is present.

See the following sample: https://github.com/marcusdacoregio/ldap-odm-index-bug

This is strange behavior that could be fixed or at least documented properly.

Related to #315

[jzheaux]

What I'm inclined to do is error if the names don't match. For example, if I do:

@DnAttribute(name="uid", index=1)

then if "uid" is at index 2, then it will apply what is in index 2 to "uid`", which is quite an unexpected result.

Alternatively, the code could error if name and index are both supplied.

I've opened #940 to address this via documentation for now since this change will likely not be passive.