JIRA Sprint Health Gadget shows "The resource /<context_path>/rest/webResources/1.0/resources, cannot be found" in a Confluence page

Problem description

If an applinked JIRA contains a context path in its base URL, the Sprint Health Gadget will display an error if embedded in a Confluence page. This bug is similar to CONFSERVER-38277 except reversed: the problem appears for a JIRA gadget when embedded in Confluence.

Steps to reproduce

1. Install Confluence 5.10.8 + JIRA 7.1.1. JIRA must have a context path in server.xml, for example "/foo".
2. Create an applink between the two
3. Add the JIRA feed to Confluence Admin > External Gadgets > Gadget Feed
4. Create a page in Confluence and add the Sprint Health Gadget from JIRA
5. Proceed through with the OAuth authentication process (i.e. "Login & Approve")

Expected behavior

The gadget does not display any errors in Confluence

Actual behavior

The gadget will display an error in a red box reflecting the context path of JIRA. For example if the context path is "/foo", it will show this on the screen:

The resource /foo/rest/webResources/1.0/resources, cannot be found.