FileUpload: use latin-1 as charset when nothing is passed.

Without this change, some tests (and possibly live code) in Plone fail because they do not pass the new required `charset` parameter.
See plone/buildout.coredev#844
And see discussion that I just started here:
#1094 (comment)

Also fixes an unclosed file in a test.

src/ZPublisher/HTTPRequest.py

@@ -1483,12 +1483,17 @@ class FileUpload:
     # that protected code can use DTML to work with FileUploads.
     __allow_access_to_unprotected_subobjects__ = 1
 
-    def __init__(self, aFieldStorage, charset):
+    def __init__(self, aFieldStorage, charset="latin-1"):
         self.file = aFieldStorage.file
         self.headers = aFieldStorage.headers
-        self.filename = aFieldStorage.filename\
-            .encode("latin-1").decode(charset)
-        self.name = aFieldStorage.name.encode("latin-1").decode(charset)
+        # Prevent needless encode-decode when both charsets are the same.
+        if charset != "latin-1":
+            self.filename = aFieldStorage.filename\
+                .encode("latin-1").decode(charset)
+            self.name = aFieldStorage.name.encode("latin-1").decode(charset)
+        else:
+            self.filename = aFieldStorage.filename
+            self.name = aFieldStorage.name
 
         # Add an assertion to the rfc822.Message object that implements
         # self.headers so that managed code can access them.

src/ZPublisher/tests/testHTTPRequest.py

@@ -845,6 +845,7 @@ def test_processInputs_w_large_input_gets_tempfile(self):
         f = req.form.get('largefile')
         self.assertTrue(f.name)
         self.assertEqual(4006, len(f.file.read()))
+        f.file.close()
 
     def test_processInputs_with_file_upload_gets_iterator(self):
         # checks fileupload object supports the iterator protocol