fix: generate code: curl: use --data-urlencode

[cvmocanu]

When the body is "Form URL encoded", the generated curl code should:

• either manually encode the value of the field
• or use --data-urlencode which does the encoding itself (not supported on old versions of curl)

Currently, if we have a field called field with a value a&=b, the value is sent as-is.

With data, the following is sent (obviously incorrect): field=a&=b
With --data-urlencode, the following is sent (correct): field=a%26%3Db

[cvmocanu]

probably this is the root cause: Kong/httpsnippet#260