Re-use HTTP Client

[thinkingserious]

Issue Summary

See https://github.com/sendgrid/csharp-http-client/pull/18/files
https://github.com/sendgrid/sendgrid-csharp/blob/master/src/SendGrid/SendGridClient.cs#L315

[jonnybee]

Do you accept Pull Requests?

[thinkingserious]

Yes we do @jonnybee :)

We will just need a signed CLA.

And a pull request would push this issue much higher in our backlog.

[thinkingserious]

@jonnybee,

We have received your CLA, thanks!