[2 of 2]: filter::parse_spec() ignore bogon empty, blank substrings

Fix issue demonstrated by unit tests in commit 2ff2bf1.

While parsing comma-separated substrings, parse_spec() now trims the
values to ensure empty and blank substrings (which are invalid) are
ignored.

Previously, only-comma-separated empty substrings were being
ignored. This change extends that intent to also ignore the empty
string (no comma), stand-alone blank strings (no comma), and comma-
separated variations of empty and/or blank substrings.

src/filter/mod.rs

@@ -299,7 +299,7 @@ fn parse_spec(spec: &str) -> (Vec<Directive>, Option<inner::Filter>) {
         return (dirs, None);
     }
     if let Some(m) = mods {
-        for s in m.split(',') {
+        for s in m.split(',').map(|ss| ss.trim()) {
             if s.is_empty() {
                 continue;
             }