Fix for Issue #1258 (#1298)

* 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
diff --git a/docs/changelog.rst b/docs/changelog.rst
index eb25578..606be41 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -47,6 +47,9 @@
 * ``pybind11_add_module()``: allow including Python as a ``SYSTEM`` include path.
   `#1416 <https://github.com/pybind/pybind11/pull/1416>`_.
 
+* ``pybind11/stl.h`` does not convert strings to ``vector<string>`` anymore.
+  `#1258 <https://github.com/pybind/pybind11/issues/1258>`_.
+
 v2.2.4 (September 11, 2018)
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