Restore Python 2.1 StringIO.py behaviour: support concatenating

Unicode string snippets to larger Unicode strings.

This fix should also go into Python 2.2.1.

Lib/StringIO.py

@@ -28,7 +28,7 @@
   bytes that occupy space in the buffer.
 - There's a simple test set (see end of this file).
 """
-
+import types
 try:
     from errno import EINVAL
 except ImportError:
@@ -38,8 +38,10 @@
 
 class StringIO:
     def __init__(self, buf = ''):
-        # Force self.buf to be a string
-        self.buf = str(buf)
+        # Force self.buf to be a string or unicode
+        if type(buf) is not types.UnicodeType:
+            buf = str(buf)
+        self.buf = buf
         self.len = len(buf)
         self.buflist = []
         self.pos = 0
@@ -135,8 +137,9 @@ def write(self, s):
         if self.closed:
             raise ValueError, "I/O operation on closed file"
         if not s: return
-        # Force s to be a string
-        s = str(s)
+        # Force s to be a string or unicode
+        if type(s) is not types.UnicodeType:
+            s = str(s)
         if self.pos > self.len:
             self.buflist.append('\0'*(self.pos - self.len))
             self.len = self.pos