fix(go): use non-pointer receiver for JSON marshal

In the case of a required anyOf property, JSON marshalling would has
been incorrect.

Required properties are not nullable, and thus always use value
receivers. For the single case of anyOf models, a pointer receiver was
used for MarshalJSON. All other instances of json marshalling use value
receivers.

This change is simply to use a value receiver instead of a pointer
receiver in the template for `MarshalJSON` on anyOf models.

Author: perhallgren

modules/openapi-generator/src/main/resources/go/model_anyof.mustache

@@ -68,7 +68,7 @@ func (dst *{{classname}}) UnmarshalJSON(data []byte) error {
 }
 
 // Marshal data from the first non-nil pointers in the struct to JSON
-func (src *{{classname}}) MarshalJSON() ([]byte, error) {
+func (src {{classname}}) MarshalJSON() ([]byte, error) {
 {{#anyOf}}
 	if src.{{#lambda.type-to-name}}{{{.}}}{{/lambda.type-to-name}} != nil {
 		return json.Marshal(&src.{{#lambda.type-to-name}}{{{.}}}{{/lambda.type-to-name}})