Add support for Sendgrid spam check

[nickrivadeneira]

Spam reports were added in thoughtbot/griddler#283. This PR:

• Adds support for Sendgrid's spam report format.
• Adds an integration test to verify that enabling spam checks no longer breaks integration with Griddler.
• Fixes broken test suite by reverting adapter changes in #26, but keeping test to verify UTF-8 encoding still works.

[parterburn]

@nickrivadeneira I know this one is old, but really curious if you know if this PR makes this statement on the griddler-sendgrid gem no longer valid (specifically the part about Spam Check):

Make sure to uncheck the "Spam Check" and "Send Raw" checkboxes on the Parse Webhook settings page, otherwise the returned parsed email will have the body stripped out.

I have Spam Check unchecked given this pretty intense warning in the README, but curious if it's all groovy now with your changes. E.g. are there any downsides to checking that box in SendGrid these days?

[nickrivadeneira]

@parterburn Sorry, I barely recall writing this 😆. What I will say is that I believe I did have Spam Check enabled and it worked fine. One way to test would be to remove the spam_report hash from the normalized params and re-run the email_spec.rb test.