Refactor WebVTT building logic into WebVTT::Builder (#4070)

spec/helpers/vtt/builder_spec.cr

@@ -0,0 +1,64 @@
+require "../../spec_helper.cr"
+
+MockLines = [
+  {
+    "start_time": Time::Span.new(seconds: 1),
+    "end_time":   Time::Span.new(seconds: 2),
+    "text":       "Line 1",
+  },
+
+  {
+    "start_time": Time::Span.new(seconds: 2),
+    "end_time":   Time::Span.new(seconds: 3),
+    "text":       "Line 2",
+  },
+]
+
+Spectator.describe "WebVTT::Builder" do
+  it "correctly builds a vtt file" do
+    result = WebVTT.build do |vtt|
+      MockLines.each do |line|
+        vtt.cue(line["start_time"], line["end_time"], line["text"])
+      end
+    end
+
+    expect(result).to eq([
+      "WEBVTT",
+      "",
+      "00:00:01.000 --> 00:00:02.000",
+      "Line 1",
+      "",
+      "00:00:02.000 --> 00:00:03.000",
+      "Line 2",
+      "",
+      "",
+    ].join('\n'))
+  end
+
+  it "correctly builds a vtt file with setting fields" do
+    setting_fields = {
+      "Kind"     => "captions",
+      "Language" => "en",
+    }
+
+    result = WebVTT.build(setting_fields) do |vtt|
+      MockLines.each do |line|
+        vtt.cue(line["start_time"], line["end_time"], line["text"])
+      end
+    end
+
+    expect(result).to eq([
+      "WEBVTT",
+      "Kind: captions",
+      "Language: en",
+      "",
+      "00:00:01.000 --> 00:00:02.000",
+      "Line 1",
+      "",
+      "00:00:02.000 --> 00:00:03.000",
+      "Line 2",
+      "",
+      "",
+    ].join('\n'))
+  end
+end