Fix "ValueError: seek of closed file" bug - closes jschneier#382

This is @mannpy's solution from:
jschneier#382 (comment)

storages/backends/s3boto3.py

@@ -436,20 +436,37 @@ def _open(self, name, mode='rb'):
         return f
 
     def _save(self, name, content):
-        cleaned_name = self._clean_name(name)
-        name = self._normalize_name(cleaned_name)
-        params = self._get_write_parameters(name, content)
-
-        if (self.gzip and
-                params['ContentType'] in self.gzip_content_types and
-                'ContentEncoding' not in params):
-            content = self._compress_content(content)
-            params['ContentEncoding'] = 'gzip'
-
-        obj = self.bucket.Object(name)
+        """
+        We create a clone of the content file as when this is passed to
+        boto3 it wrongly closes the file upon upload where as the storage
+        backend expects it to still be open
+        """
+        # Seek our content back to the start
         content.seek(0, os.SEEK_SET)
-        obj.upload_fileobj(content, ExtraArgs=params)
-        return cleaned_name
+
+        # Create a temporary file that will write to disk after a specified
+        # size. This file will be automatically deleted when closed by
+        # boto3 or after exiting the `with` statement if the boto3 is fixed
+        with SpooledTemporaryFile() as content_autoclose:
+            # Write our original content into our copy that will be closed by boto3
+            content_autoclose.write(content.read())
+
+            # Upload the object which will auto close the
+            # content_autoclose instance
+            cleaned_name = self._clean_name(name)
+            name = self._normalize_name(cleaned_name)
+            params = self._get_write_parameters(name, content)
+
+            if (self.gzip and
+                    params['ContentType'] in self.gzip_content_types and
+                    'ContentEncoding' not in params):
+                content = self._compress_content(content)
+                params['ContentEncoding'] = 'gzip'
+
+            obj = self.bucket.Object(name)
+            content.seek(0, os.SEEK_SET)
+            obj.upload_fileobj(content, ExtraArgs=params)
+            return cleaned_name
 
     def delete(self, name):
         name = self._normalize_name(self._clean_name(name))