Fix debugging output for nameless py::arg_v annotations (#648)

* Fix debugging output for nameless py::arg annotations

This fixes a couple bugs with nameless py::arg() (introduced in #634)
annotations:

- the argument name was being used in debug mode without checking that
  it exists (which would result in the std::string construction throwing
  an exception for being invoked with a nullptr)
- the error output says "keyword arguments", but py::arg_v() can now
  also be used for positional argument defaults.
- the debugging output "in function named 'blah'" was overly verbose:
  changed it to just "in function 'blah'".

* Fix missing space in debug test string

* Moved tests from issues to methods_and_attributes