Check for JSON before parsing JSON in errors

The createError function was incorrectly assuming that response
bodies are in JSON format and attempting to extract useful details
from that body.  But, for some error cases, the body is not JSON
resulting in a error being thrown during the process of adding
details to the error.

This fix adds safeguards around the places that need JSON to enable
the createError method to extract the useful details.

src/OAuthClient.js

@@ -585,7 +585,7 @@ OAuthClient.prototype.createError = function createError(e, authResponse) {
   e.originalMessage = e.message;
 
   e.error = '';
-  if ('error' in authResponse.getJson()) {
+  if (authResponse.isJson() && 'error' in authResponse.getJson()) {
     e.error = authResponse.getJson().error;
   } else if (authResponse.response.statusText) {
     e.error = authResponse.response.statusText;
@@ -594,7 +594,7 @@ OAuthClient.prototype.createError = function createError(e, authResponse) {
   }
 
   e.error_description = '';
-  if ('error_description' in authResponse.getJson()) {
+  if (authResponse.isJson() && 'error_description' in authResponse.getJson()) {
     e.error_description = authResponse.getJson().error_description;
   } else if (authResponse.response.statusText) {
     e.error_description = authResponse.response.statusText;