Fix rust fenced code blocks with an indent

This fixes a bug in the Rust code block partitioning that was
incorrectly removing the whitespace from the beginning of a code block.

crates/mdbook-html/src/html/hide_lines.rs

@@ -141,8 +141,15 @@ fn partition_rust_source(s: &str) -> (&str, &str) {
     let split_idx = match HEADER_RE.captures(s) {
         Some(caps) => {
             let attributes = &caps[1];
+            if attributes.trim().is_empty() {
+                // Don't include pure whitespace as an attribute. The
+                // whitespace in the regex is intended to handle multiple
+                // attributes *separated* by potential whitespace.
+                0
+            } else {
                 attributes.len()
             }
+        }
         None => 0,
     };
     s.split_at(split_idx)