Fix rustoc item summaries that are headers

Rustoc item summaries that are headers were not displayed at all because
they started with whitespace.

This PR fixes this and now removes the whitespace and then displays the
block.

src/librustdoc/html/render.rs

@@ -1819,7 +1819,9 @@ fn full_path(cx: &Context, item: &clean::Item) -> String {
 
 fn shorter<'a>(s: Option<&'a str>) -> String {
     match s {
-        Some(s) => s.lines().take_while(|line|{
+        Some(s) => s.lines()
+            .skip_while(|s| s.chars().all(|c| c.is_whitespace()))
+            .take_while(|line|{
             (*line).chars().any(|chr|{
                 !chr.is_whitespace()
             })