Add more unit tests for coverage

[chinglee-iot]

Add more unit tests

Description

• Cover on_status_complete callback added in Add on_status_complete callback #173
• Cover HTTP_RESPONSE_DO_NOT_PARSE_BODY_FLAG added in Add Ability to Stop Parsing After HTTP Headers #172
• Cover HTTP_REQUEST_NO_USER_AGENT_FLAG add in [support presigned URL] support http request without HTTP_USER_AGENT_FIELD #164
• Enforce 100% code coverage in unit test

Test Steps

Running unit test the code coverage should be 100%.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

#164, #172, #173