Sprint value not exported to XML

Steps to replicate

1. Install Jira agile version 6.6.13
2. Create sample scrum board with sample data
3. Choose Issues > Search for Issues.
4. Refine your search, as described in Searching for Issues, then choose the Export menu.
5. Choose XML from the dropdown menu.
6. Find SSPA-19

Observed Behavior

<customfield id="customfield_10005" key="com.pyxis.greenhopper.jira:gh-sprint"><customfieldname>Sprint</customfieldname><customfieldvalues></customfieldvalues></customfield>

Expected Behavior

<customfield id="customfield_10000" key="com.pyxis.greenhopper.jira:gh-sprint"><customfieldname>Sprint</customfieldname><customfieldvalues><customfieldvalue>1</customfieldvalue></customfieldvalues></customfield>