formEncodeHash
Encodes @form_data_set into a value of type "application/x-www-form-urlencoded".
Encodes as defined in the HTML 4.01 spec.
Note that the HTML spec states that "The control names/values are listed in the order they appear in the document." Since this method takes a hash table, it cannot enforce that; if you care about the ordering of the form fields, use func@form_encode_datalist.
See also: ctor@Message.new_from_encoded_form.
Return
the encoded form
Parameters
formDataSet
a hash table containing name/value pairs (as strings)