Preserve whitespace when stripping the header

src/format.jl

@@ -90,7 +90,9 @@ function get_titleblock(doc::WeaveDoc)
 end
 
 function strip_header(chunk::DocChunk)
-  chunk.content = lstrip(replace(chunk.content, r"^---$(?<header>.+)^---$"ms, ""))
+  if ismatch(r"^---$(?<header>.+)^---$"ms, chunk.content)
+    chunk.content = lstrip(replace(chunk.content, r"^---$(?<header>.+)^---$"ms, ""))
+  end
   return chunk
 end
 

test/documents/multimedia/rich_output.md.ref

@@ -1,4 +1,5 @@
 
+
 ~~~~{.julia}
 using DataFrames
 df = DataFrame(letters = 'a':'z', numbers = 1:26)

test/documents/multimedia/rich_output.tex.ref

@@ -1,4 +1,5 @@
 
+
 \begin{juliacode}
 using DataFrames
 df = DataFrame(letters = 'a':'z', numbers = 1:26)