Added support of replyToList in the library

[subinoy7]

Implementation of sendgrid#339 & sendgrid#1301
The replyToList support was added in v3 API but it wasn't supported in the library. With this changes, we can:

• Use the replyToList header to set multiple emails in the replyTo
• Error handling are placed if the format is not in acceptable
• Fixed old test cases that were failing as SendGrid changed the acceptable response code to 202

Acceptable Format:
[ { "name": "Test Team", "email": "test@example.org" }, { "name": "Support Test Team", "email": "support.test@example.org" } ]

If any one of the records doesn't have email filed, then it will return an error. Also the value of replyToList should be an array.