[go-server] Set default values in object properties

When a default value is set for an object property, ensure it is set
into the struct before decoding the JSON body.

Fix #4579

Author: rledisez

modules/openapi-generator/src/main/resources/go-server/controller-api.mustache

@@ -588,6 +588,11 @@ func (c *{{classname}}Controller) {{nickname}}(w http.ResponseWriter, r *http.Re
 	{{/isHeaderParam}}
 	{{#isBodyParam}}
 	{{paramName}}Param := {{dataType}}{}
+	{{#vars}}
+	{{#defaultValue}}
+	{{paramName}}Param.{{^isArray}}{{#isBoolean}}{{name}} = {{{.}}}{{/isBoolean}}{{#isNumeric}}{{name}} = {{{.}}}{{/isNumeric}}{{^isBoolean}}{{^isNumeric}}{{name}} = "{{{.}}}"{{/isNumeric}}{{/isBoolean}}{{/isArray}}
+	{{/defaultValue}}
+	{{/vars}}
 	d := json.NewDecoder(r.Body)
 	{{^isAdditionalPropertiesTrue}}
 	d.DisallowUnknownFields()