Issue #1548891: The cStringIO.StringIO() constructor now encodes unicode
arguments with the system default encoding just like the write() method
does, instead of converting it to a raw buffer.
diff --git a/Modules/cStringIO.c b/Modules/cStringIO.c
index be9ad50..89f1dd6 100644
--- a/Modules/cStringIO.c
+++ b/Modules/cStringIO.c
@@ -661,7 +661,11 @@
   char *buf;
   Py_ssize_t size;
 
-  if (PyObject_AsReadBuffer(s, (const void **)&buf, &size)) {
+  if (PyUnicode_Check(s)) {
+    if (PyObject_AsCharBuffer(s, (const char **)&buf, &size) != 0)
+      return NULL;
+  }
+  else if (PyObject_AsReadBuffer(s, (const void **)&buf, &size)) {
     PyErr_Format(PyExc_TypeError, "expected read buffer, %.200s found",
                  s->ob_type->tp_name);
     return NULL;