Make the markdown filter compatible with String

This commit solves issue #719.

This is done by making the markdown filter borrow the string and
simplifying the filter to accept `&str` instead of `AsRef<str>`

Add test for the markdown filter using  as input

Revert markdown filter changes

Revert unnecessary changes

Improve test_markdown_owned_string test

Use cargo fmt

askama_derive/src/generator.rs

@@ -1200,7 +1200,7 @@ impl<'a> Generator<'a> {
         };
 
         buf.write(&format!(
-            "::askama::filters::markdown({}, ",
+            "::askama::filters::markdown({}, &",
             self.input.escaper
         ));
         self.visit_expr(buf, md)?;

testing/tests/markdown.rs

@@ -70,3 +70,21 @@ before\
 after",
     );
 }
+
+#[derive(Template)]
+#[template(source = "{{content|markdown}}", ext = "html")]
+struct MarkdownStringTemplate {
+    content: String,
+}
+
+// Tests if the markdown filter accepts String
+#[test]
+fn test_markdown_owned_string() {
+    let template = MarkdownStringTemplate {
+        content: "The markdown filter _indeed_ works with __String__".into(),
+    };
+    assert_eq!(
+        template.render().unwrap(),
+        "<p>The markdown filter <em>indeed</em> works with <strong>String</strong></p>\n"
+    )
+}