upgrade to mvel 2.1.5
update to joda 2.2
resolved empty setting values should be removed
when resolving empty settings values, their value should be removed, for example, when using ${env.ENV_VAR}, and ENV_VAR is not set, then the setting should be removed
Allow to set headers in HTTP response

This commit allows to set custom headers in HTTP responses (like
setting the WWW-Authenticate header for basic auth) by adding
RestRequest.addHeader() method.

Closes #2936
Closes #2540

To get the history right: This is based on PR #2723