Sendgrid

Fixed
- Issue 321: Installing 4.2.0 installs the wrong version of python-http-client
- Big thanks to [w-](https://github.com/w-) for the heads up!

Added
- Pull 318 Add ability to reset request headers on client attribute
- Big thanks to [w-](https://github.com/w-) for the pull request!

Added
- Pull 314 Add ability to impersonate subuser
- Big thanks to [w-](https://github.com/w-) for the pull request!

BREAKING CHANGE
- Pull 244 [refactor helpers using property getter/setter](https://github.com/sendgrid/sendgrid-python/pull/244/files)
- Big thanks to [Denis Vlasov](https://github.com/denis90) for the pull request!
- The changes break the implementation of the [Mail Helper](sendgrid/helpers/mail) `Mail()` class
- `set_from()` is now the property `from_email`
- `set_subject()` is now the property `subject`
- `set_template_id()` is now the property `template_id`
- `set_send_at()` is now the property `send_at`
- `set_batch_id()` is now the property `batch_id`
- `set_asm()` is now the property `asm`
- `set_ip_pool_name()` is now the property `ip_pool_name`
- `set_mail_settings()` is now the property `mail_settings`
- `set_tracking_settings()` is now the property `tracking_settings`
- `set_reply_to()` is now the property `reply_to`
- `personalization.set_send_at()` is now the property `personalization.send_at`
- `personalization.set_subject()` is now the property `personalization.subject`
- `attachment.set_content()` is now the property `attachment.content`
- `attachment.set_type()` is now the property `attachment.type`
- `attachment.set_filename()` is now the property `attachment.filename`
- `attachment.set_disposition()` is now the property `attachment.disposition`
- `attachment.set_content_id()` is now the property `attachment.content_id`
- `mail_settings.set_bcc_settings()` is now the property `mail_settings.bcc_settings`
- `mail_settings.set_bypass_list_management()` is now the property `mail_settings.bypass_list_management`
- `mail_settings.set_footer_settings()` is now the property `mail_settings.footer_settings`
- `mail_settings.set_sandbox_mode()` is now the property `mail_settings.sandbox_mode`
- `mail_settings.set_spam_check()` is now the property `mail_settings.spam_check`
- `tracking_settings.set_click_tracking()` is now the property `click_tracking`
- `tracking_settings.set_open_tracking()` is now the property `open_tracking`
- `tracking_settings.set_subscription_tracking()` is now the property `subscription_tracking`
- `tracking_settings.set_ganalytics()` is now the property `ganalytics`

Updated
- Pull 300 [Exclude test package](https://github.com/sendgrid/sendgrid-python/pull/250)
- Big thanks to [Iryna Shcherbina](https://github.com/irushchyshyn) for the pull request!

Updated
- Pull 250 [Improve code quality](https://github.com/sendgrid/sendgrid-python/pull/250)
- Big thanks to [Andrii Soldatenko](https://github.com/andriisoldatenko) for the pull request!