Fix for #1313 & #1332

For issue #1313, partial fix: enhance coding/deconding to missing part.
Now filename is correctly undecoded. 
Name and Value must be encoded to allow parsing and respect to HTTP RFC since all value and name must be URI encoded (and then decoded). Some items were not correctly decoded, so the reason of the issue.

For issue #1332, add text/plain to content-type as default type if not specified.