md: emit newline before closing fence in codeBlock

The codeBlock renderer wrote the closing backtick fence directly after
the node literal. When the literal did not end with a newline (as with
a manually constructed ast.CodeBlock), the result was malformed markdown
like 'val x : Int = 42```' on a single line.

Emit a newline before the closing fence when the literal does not
already end with one. Also correct the expected string in
TestRenderCodeBlock to end with \n\n (matching doubleSpace semantics
used by every other test in this file).

Author: kardamanov

md/md_renderer.go

@@ -264,6 +264,9 @@ func (r *Renderer) codeBlock(w io.Writer, node *ast.CodeBlock) {
 	}
 	r.outs(w, "\n")
 	r.out(w, text)
+	if len(text) == 0 || text[len(text)-1] != '\n' {
+		r.outs(w, "\n")
+	}
 	r.outs(w, "```\n\n")
 }
 

md/md_renderer_test.go

@@ -73,7 +73,7 @@ func TestRenderCode(t *testing.T) {
 func TestRenderCodeBlock(t *testing.T) {
 	input := &ast.CodeBlock{Info: []byte(string("scala"))}
 	input.Literal = []byte(string("val x : Int = 42"))
-	expected := "\n```scala\nval x : Int = 42\n```\n"
+	expected := "\n```scala\nval x : Int = 42\n```\n\n"
 	testRendering(t, input, expected)
 }