FIX: Hide excerpt of binary files in GitHub onebox (#15639)

Oneboxer did not know if a file is binary or not and always tried to
show an excerpt of the file.

spec/lib/onebox/engine/github_blob_onebox_spec.rb

@@ -5,6 +5,7 @@ require "rails_helper"
 describe Onebox::Engine::GithubBlobOnebox do
   before do
     @link = "https://github.com/discourse/onebox/blob/master/lib/onebox/engine/github_blob_onebox.rb"
+    @uri = URI.parse(@link)
     stub_request(:get, "https://raw.githubusercontent.com/discourse/onebox/master/lib/onebox/engine/github_blob_onebox.rb")
       .to_return(status: 200, body: onebox_response(described_class.onebox_name))
   end
@@ -20,5 +21,16 @@ describe Onebox::Engine::GithubBlobOnebox do
     it "includes blob contents" do
       expect(html).to include("module Oneboxer")
     end
+
+    it "does not include blob contents if it is binary" do
+      # stub_request if the response must be binary (ASCII-8BIT)
+      uri = mock('object')
+      uri.stubs(:open).returns(File.open("#{Rails.root}/spec/fixtures/pdf/small.pdf", "rb"))
+      URI.stubs(:parse).with(@link).returns(@uri)
+      URI.stubs(:parse).with('https://raw.githubusercontent.com/discourse/onebox/master/lib/onebox/engine/github_blob_onebox.rb').returns(uri)
+
+      expect(html).not_to include('/Pages')
+      expect(html).to include('This file is binary.')
+    end
   end
 end