Update flag when defaultCountry value is changed

[dhanesh-kapadiya]

This PR adds functionality of updating country flag when defaultCountry props value is updated after component is mounted.

Description

Current version of the project doesn't support the functionality of updating flag when value passed in props defaultCountry is changed.
To support this functionality, I have added a function named updateFlagOnDefaultCountryChange which will pass the value of this.props.defaultCountry as CountryCode in setFlag function.

Event componentDidUpdate determines whether this function should be called or not based on current and previous value of props defaultCountry.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have used ESLint & Prettier to follow the code style of this project.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Coverage decreased (-0.3%) to 92.953% when pulling 0336b10 on dhanesh-kapadiya:master into 3f9b3c7 on patw0929:master.

[patw0929]

Merged to master manually and published in v7.0.3.