fix: Remove leading whitespace from Subject (#5106)

If Subject is multiline-formatted, `mailparse` adds the leading whitespace to it. The solution is to
always remove the leading whitespace, because if Subject isn't multiline-formatted, it never
contains the leading whitespace anyway. But as for the trailing whitespace -- i checked -- it's
never removed, so let's keep this as is.

src/mimeparser.rs

@@ -809,6 +809,7 @@ impl MimeMessage {
 
     pub(crate) fn get_subject(&self) -> Option<String> {
         self.get_header(HeaderDef::Subject)
+            .map(|s| s.trim_start())
             .filter(|s| !s.is_empty())
             .map(|s| s.to_string())
     }