Test for flex message

[Yang-33]

though there are similar tests, there are no tests yet that verify the value after serialization (to_json), so this change adds it.

[Yang-33]

     
     WebMock::NetConnectNotAllowedError:
       Real HTTP connections are disabled. Unregistered request: POST https://api.line.me/v2/bot/message/broadcast with body '{"messages":[{"type":"flex","alt_text":"Test Alt Text","contents":{"type":"bubble","direction":"ltr","body":{"type":"text","text":"Test Text","size":"xl","weight":"bold"}}}],"notificationDisabled":false}' with headers {'Accept'=>'*/*', 'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3', 'Authorization'=>'Bearer test-channel-access-token', 'Content-Type'=>'application/json', 'Host'=>'api.line.me', 'User-Agent'=>'LINE-BotSDK-Ruby/2.0.0'}
     
       You can stub this request with the following snippet:
     
       stub_request(:post, "https://api.line.me/v2/bot/message/broadcast").
         with(
           body: "{\"messages\":[{\"type\":\"flex\",\"alt_text\":\"Test Alt Text\",\"contents\":{\"type\":\"bubble\",\"direction\":\"ltr\",\"body\":{\"type\":\"text\",\"text\":\"Test Text\",\"size\":\"xl\",\"weight\":\"bold\"}}}],\"notificationDisabled\":false}",
           headers: {
          'Accept'=>'*/*',
          'Accept-Encoding'=>'gzip;q=1.0,deflate;q=0.6,identity;q=0.3',
          'Authorization'=>'Bearer test-channel-access-token',
          'Content-Type'=>'application/json',
          'Host'=>'api.line.me',
          'User-Agent'=>'LINE-BotSDK-Ruby/2.0.0'
           }).
         to_return(status: 200, body: "", headers: {})
     
       registered request stubs:
     
       stub_request(:post, "https://api.line.me/v2/bot/message/broadcast").
         with(
           body: "{\"messages\":[{\"type\":\"flex\",\"altText\":\"Test Alt Text\",\"contents\":{\"type\":\"bubble\",\"direction\":\"ltr\",\"body\":{\"type\":\"text\",\"text\":\"Test Text\",\"size\":\"xl\",\"weight\":\"bold\"}}}],\"notificationDisabled\":false}",
           headers: {
          'Authorization'=>'Bearer test-channel-access-token'
           })
     
       Body diff:
        [["-", "messages[0].alt_text", "Test Alt Text"],
        ["+", "messages[0].altText", "Test Alt Text"]]

alt_text in Line::Bot::V2::MessagingApi::FlexMessage sends as alt_text(wrong), not altText(correct)

[Yang-33]

it's related to #448 ..

[Yang-33]

#455 fixed this, thank you!

[Yang-33]

#365