Example (#4730)

* add tests for example feature
* refactor ParameterRow value setter logic
* aside: fix property access in sampleFromSchema
* prioritize media type examples for OAS3 responses
* use `example` in schema level example
* refactor: move stringify to utils
* prioritize media type examples in OAS3 request bodies
* modify nightwatch config
* fix parameter/response regressions
* reorder and broaden default value sources
* update lockfile

src/core/plugins/samples/fn.js

@@ -52,10 +52,10 @@ export const sampleFromSchema = (schema, config={}) => {
     let props = objectify(properties)
     let obj = {}
     for (var name in props) {
-      if ( props[name].readOnly && !includeReadOnly ) {
+      if ( props[name] && props[name].readOnly && !includeReadOnly ) {
         continue
       }
-      if ( props[name].writeOnly && !includeWriteOnly ) {
+      if ( props[name] && props[name].writeOnly && !includeWriteOnly ) {
         continue
       }
       obj[name] = sampleFromSchema(props[name], config)