Add test codes for json serialize of model

[be-hase]

To prevent like a this issues.
#111

[tkgauri]

Need to add test cases for models present in line-bot-sdk-python/linebot/models/
Nice to have!

[okue]

test_messages.py is added in directory tests/models/ (#177)

[okue]

Json-serializing-test should be added for the following modules:

• actions.py
• flex_message.py
• imagemap.py
• send_messages.py
• template.py