recipe/simplehttpserver: fix application/x-www-form-urlencoded POST on py3

In deploy test, this recipe is used with url encoded POST [1], but the
test only tested multipart encoded POSTs. In python 3, they are different,
with multipart POSTs, FieldStorage values are bytes, but with url encoded
POSTs, the values are string.

This small server from the recipe only supported multipart POSTs, this
change add support for url encoded POSTs.

[1]: https://lab.nexedi.com/nexedi/slapos/blob/dd7038feab70866e54028de89d0e0e1017fc276c/software/erp5testnode/testsuite/deploy-test/deploy-script-controller#L86