collect-sling-repos.groovy: 'Link' header is now lowercase

Not sure how or why this broke, but it looks safer to check for both
upper-case and lower-case first letter.
diff --git a/collect-sling-repos.groovy b/collect-sling-repos.groovy
index 802c551..cf4f293 100755
--- a/collect-sling-repos.groovy
+++ b/collect-sling-repos.groovy
@@ -55,7 +55,7 @@
     // find link to next page, if applicable
     link = null
     
-    def links = conn.headerFields['Link']
+    def links = conn.headerFields['Link'] ?: conn.headerFields['link']
     if ( links ) {
         def next = links[0].split(',').find{ it.contains('rel="next"') }
         link = next != null ? next.find('<(.*)>').replaceAll('<|>',''): null