Improve firefox profile prefs merging (#423)

Merges prefs::Default prefs into custom profile unless the custom
profile explicitly sets that preference.

Sets the marionette.defaultPrefs.port preference last so users cannot
accidentally overwrite its value by providing it in capabilities.

src/marionette.rs

@@ -343,11 +343,12 @@ impl MarionetteHandler {
                          .map_err(|_| WebDriverError::new(ErrorStatus::UnknownError,
                                                           "Unable to read profile preferences file")));
 
-        prefs.insert("marionette.defaultPrefs.port", Pref::new(port as i64));
+        for &(ref name, ref value) in prefs::DEFAULT.iter() {
+            if !custom_profile || !prefs.contains_key(name) {
+                prefs.insert((*name).clone(), (*value).clone());
+            }
+        }
 
-        if !custom_profile {
-            prefs.insert_slice(&prefs::DEFAULT[..]);
-        };
         prefs.insert_slice(&extra_prefs[..]);
 
         prefs.insert_slice(&prefs::REQUIRED[..]);
@@ -356,6 +357,8 @@ impl MarionetteHandler {
             prefs.insert("marionette.logging", Pref::new(level.to_string()));
         };
 
+        prefs.insert("marionette.defaultPrefs.port", Pref::new(port as i64));
+
         prefs.write().map_err(|_| WebDriverError::new(ErrorStatus::UnknownError,
                                                       "Unable to write Firefox profile"))
     }