Added a test for the StringIO write() error I just fixed.

Lib/test/output/test_StringIO

@@ -3,6 +3,7 @@ abcdefghij
 klmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
 
 2
+'abcuvwxyz!'
 'abcdefghij'
 'abcde'
 Caught expected ValueError writing to closed StringIO:
@@ -11,6 +12,7 @@ abcdefghij
 klmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
 
 2
+'abcuvwxyz!'
 'abcdefghij'
 'abcde'
 Caught expected ValueError writing to closed StringIO:

Lib/test/test_StringIO.py

@@ -7,6 +7,13 @@ def do_test(module):
     print f.readline()
     print len(f.readlines(60))
 
+    f = module.StringIO()
+    f.write('abcdef')
+    f.seek(3)
+    f.write('uvwxyz')
+    f.write('!')
+    print `f.getvalue()`
+    f.close()
     f = module.StringIO()
     f.write(s)
     f.seek(10)