* Fix for Issue #1258
list_caster::load method will now check for a Python string and prevent its automatic conversion to a list.
This should fix the issue "pybind11/stl.h converts string to vector<string> #1258" (https://github.com/pybind/pybind11/issues/1258)
* Added tests for fix of issue #1258
* Changelog: stl string auto-conversion
