Async await fix

[thinkingserious]

Issue Summary

A fix for this thread: sendgrid/sendgrid-csharp#259 (comment)

Steps to Reproduce

It appears that this call to .Result is misplaced: https://github.com/sendgrid/csharp-http-client/blob/master/CSharpHTTPClient/Client.cs#L245

Technical details:

• csharp-http-client Version: 2.0.2
• CSharp Version: 4.5.2