meetup_rss(description_header): Improve string join section

Instead of the verbose and unclear appending, use a `join` to add the
newline. Filter out empty strings to ensure the newline doesn't get
added if any of `where` or `place` are empty.

Needed to convert the slice strings to owned strings to deal with this
error:

    error[E0599]: no method named `join` found for type `std::vec::Vec<&std::string::String>` in the current scope
      --> src/meetup_rss.rs:92:10
       |
    92 |         .join("\n")
       |          ^^^^

1 files changed, 6 insertions, 11 deletions

diff --git a/src/meetup_rss.rs b/src/meetup_rss.rs
index efdaa5e..80409f9 100644
--- a/src/meetup_rss.rs
+++ b/src/meetup_rss.rs
@@ -73,17 +73,12 @@ fn description_header(event: &Event) -> String {
         "".to_owned()
     };
 
-    let header = when;
-
-    let header = if !header.is_empty() && !place.is_empty() {
-        format!("{}\n", header)
-    } else {
-        header
-    };
-
-    let header = format!("{}{}", header, place);
-
-    header
+    [when, place]
+        .iter()
+        .filter(|s| !s.is_empty())
+        .map(|s| s.to_owned())
+        .collect::<Vec<String>>()
+        .join("\n")
 }