feat: Avoid defaulting to English if locale is not set

If `locale" is not specified, we prefer to honor an Embedded user's default browser language setting (if it is supported by our web app) instead of always forcing users into viewing our web app in English. Our backend now allows the `user_culture` param to be optional, and if it is blank or omitted then it will tell the web app to prefer the user's default browser language.

Also in this feat:
- Update tests
- Simplify test mock GUIDs
- Correct a few comments

Author: nathanbuchar

src/defaults.js

@@ -1,9 +1,6 @@
-import settings from './settings';
-
 export default {
   allowCancel: true,
   debug: false,
-  locale: settings.locales.EN_US,
   skipDomainVerification: false,
   testMode: false,
   timeout: 30000, // 30 seconds

src/embedded.js

@@ -252,21 +252,23 @@ class HelloSign extends Emitter {
   }
 
   /**
-   * Validates and appends the "user_culture" parameter to
-   * the iFrame params object.
+   * Appends the "user_culture" parameter to the iFrame
+   * params object if it is defined.
    *
-   * @throws {TypeError} if locale is not a string
    * @param {URLSearchParams} params
    * @private
    */
   _applyLocale(params) {
     const val = this._config.locale;
 
-    if (typeof val !== 'string') {
-      throw new TypeError('"locale" must be a string');
+    // If "locale" is not defined, then the "user_culture"
+    // param is not sent to the app. This tells the app to
+    // try use the user's default browser language, if it
+    // is supported by HelloSign. Otherwise, the app falls
+    // back to English.
+    if (val && val.length) {
+      params.append('user_culture', val);
     }
-
-    params.append('user_culture', val);
   }
 
   /**