Use strings instead of symbols for consistency in harvester

Both the #repositories method and the #record_repo method return
strings, so it's easier to just transform @ogm_repos to use
string keys than trying to force everything to be symbols.

lib/earthworks/harvester.rb

@@ -9,7 +9,7 @@ class Harvester < GeoCombine::Harvester
     def initialize(ogm_repos: ENV.fetch('OGM_REPOS'), **kwargs)
       super(**kwargs)
 
-      @ogm_repos = ogm_repos
+      @ogm_repos = ogm_repos.transform_keys(&:to_s)
     end
 
     # Support skipping and transforming arbitrary records prior to indexing
@@ -49,7 +49,7 @@ def record_repo(path)
 
     # Only harvest configured repositories, if configuration was provided
     def repositories
-      @repositories ||= @ogm_repos ? super.compact.select { |repo| @ogm_repos.key?(repo.to_sym) } : super
+      @repositories ||= @ogm_repos ? super.compact.select { |repo| @ogm_repos.key?(repo) } : super
     end
   end
 end