More improvements in Mailbox

[mshahidrasheed]

I have Updated this class.

The following end-points has been implemented.

• PUT https://api.pipedrive.com/v1/mailbox/mailThreads/{id}
• DELETE https://api.pipedrive.com/v1/mailbox/mailThreads/{id}
• GET https://api.pipedrive.com/v1/mailbox/mailMessages/{id}

[IsraelOrtuno]

Thanks !

[mshahidrasheed]

welcome

[mshahidrasheed]

@IsraelOrtuno kindly add me as a contributor in your repository.