[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 OpenAPITools#4579

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

@@ -591,7 +591,12 @@ func (c *{{classname}}Controller) {{nickname}}(w http.ResponseWriter, r *http.Re
 	{{paramName}}Param := r.Header.Get("{{baseName}}")
 	{{/isHeaderParam}}
 	{{#isBodyParam}}
+	{{#isArray}}
 	{{paramName}}Param := {{dataType}}{}
+	{{/isArray}}
+	{{^isArray}}
+	{{paramName}}Param := New{{dataType}}WithDefaults()
+	{{/isArray}}
 	d := json.NewDecoder(r.Body)
 	{{^isAdditionalPropertiesTrue}}
 	d.DisallowUnknownFields()

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

@@ -74,7 +74,27 @@ type {{classname}} struct {
 	{{/deprecated}}
 	{{name}} {{#isNullable}}*{{/isNullable}}{{{dataType}}} `json:"{{baseName}}{{^required}},omitempty{{/required}}"{{#vendorExtensions.x-go-custom-tag}} {{{.}}}{{/vendorExtensions.x-go-custom-tag}}`
 {{/vars}}
-}{{/isEnum}}
+}
+
+{{^isArray}}
+// New{{classname}}WithDefaults instantiates a new {{classname}} object
+// This constructor will only assign default values to properties that have it defined,
+// but it doesn't guarantee that properties required by API are set
+func New{{classname}}WithDefaults() {{classname}} {
+	this := {{classname}}{}
+{{#vars}}
+{{#defaultValue}}
+{{^isArray}}
+	this.{{name}} = {{#isBoolean}}{{{.}}}{{/isBoolean}}{{#isNumeric}}{{{.}}}{{/isNumeric}}{{^isBoolean}}{{^isNumeric}}"{{{.}}}"{{/isNumeric}}{{/isBoolean}}
+{{/isArray}}
+{{/defaultValue}}
+{{#isModel}}
+	{{#isNullable}}*{{/isNullable}}this.{{name}} = New{{dataType}}WithDefaults()
+{{/isModel}}
+{{/vars}}
+	return this
+}{{/isArray}}
+{{/isEnum}}
 
 // Assert{{classname}}Required checks if the required fields are not zero-ed
 func Assert{{classname}}Required(obj {{classname}}) error {