Fix types based on dialyzer checks

[define-null]

I found this bugs, while running dialyzer for the project that uses soap. While the change might not reflect what was the initial intention of the types for those fields - it at least reflect the current code.

So we might end up with undefined value in wrapper_ns field because of that code: https://github.com/bet365/soap/blob/master/src/soap_parse_wsdl.erl#L337 and decoders just don't have any default falue. So you might wanna change it to decoders = [] :: [{string(), module}] instead of having undefined there.

[define-null]

Added soap_body spec fix

[define-null]

Any chance to merge this?@willemdj @cmullaparthi