Github client connection is not taken outbound https proxy settings

Here is the steps to reproduce the issue:

1. Install JIRA with an outbound proxy which makes that the Internet connection only possible via proxy.
2. Try to add a new github account to JIRA.

Expected behaviour:
The DVCS plugin should connect the proxy then let the proxy to forward the connection to https://github.com.

Current behaviour:
The DVCS plugin is trying to establish a connection directly to https://github.com which failed all the time.