filter tags correctly when since_tag is set to most recent tag (#566)

* filter tags correctly when `since_tag` is set to most recent tag
Before this fix [`filtered_tags`](https://github.com/skywinder/github-changelog-generator/blob/master/lib/github_changelog_generator/generator/generator_generation.rb#L132)
was empty which caused a fallback to the last tag in the sorted_tags array (which is the oldest tag).
There for no issues and PRs were filtered in that case.

fixes #555
fixes #304

spec/unit/generator/generator_tags_spec.rb

@@ -182,6 +182,11 @@ describe GitHubChangelogGenerator::Generator do
         let(:generator) { GitHubChangelogGenerator::Generator.new(since_tag: "2") }
         it { is_expected.to be_a Array }
         it { is_expected.to match_array(tags_from_strings(%w[1 2])) }
+
+        context "with since tag set to the most recent tag" do
+          let(:generator) { GitHubChangelogGenerator::Generator.new(since_tag: "1") }
+          it { is_expected.to match_array(tags_from_strings(%w[1])) }
+        end
       end
 
       context "with invalid since tag" do