[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

@@ -587,7 +587,25 @@ func (c *{{classname}}Controller) {{nickname}}(w http.ResponseWriter, r *http.Re
 	{{paramName}}Param := r.Header.Get("{{baseName}}")
 	{{/isHeaderParam}}
 	{{#isBodyParam}}
-	{{paramName}}Param := {{dataType}}{}
+	{{paramName}}Param := {{dataType}}{
+	    {{#vars}}
+	    {{#defaultValue}}
+	    {{^isArray}}
+	    {{#isBoolean}}
+	    {{name}}: {{{.}}},
+	    {{/isBoolean}}
+	    {{#isNumeric}}
+	    {{name}}: {{{.}}},
+	    {{/isNumeric}}
+	    {{^isBoolean}}
+	    {{^isNumeric}}
+	    {{name}}: "{{{.}}}",
+	    {{/isNumeric}}
+	    {{/isBoolean}}
+	    {{/isArray}}
+	    {{/defaultValue}}
+	    {{/vars}}
+	}
 	d := json.NewDecoder(r.Body)
 	{{^isAdditionalPropertiesTrue}}
 	d.DisallowUnknownFields()