Removed references to Microsoft.AspNetCore.Http.Abstractions package …

[Jac21]

…within src\SendGrid project

Fixes #707

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

See title, removed references to AspNetCore specific package within netstandard1.3 and netstandard2.0 target frameworks.

[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Hello @Jac21,

Thanks again for the PR!

It's HACKTOBERFEST! We want to show our appreciation by sending you some special Hacktoberfest swag. If you have not already, could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX

[thinkingserious]

Hello @Jac21,

Thanks again for the PR!

We noticed that you have earned 1 out of the 5 points needed to receive glorious SendGrid Hacktoberfest swag.

Please take a moment to checkout this link to find more issues to get you past the required threshold.

Also, please be sure that you have officially registered here.

Thank you and Happy Hacktobering!

Team SendGrid DX

[Jac21]

Thanks for the heads up @thinkingserious, I'll try my best!

[thinkingserious]

Hello @Jac21,

Thanks again for the PR!

You have earned 1 out of the 5 points needed to receive glorious SendGrid Hacktoberfest swag.

Please take a moment to checkout this link to find more issues to get you past the required threshold or to simply continue the celebration.

Also, please be sure you have officially registered with us here by November 1, 2018 to qualify.

If you have any questions you can email us at dx+hacktoberfest2018@sendgrid.com.

Thank you and Happy Hacktobering!

Team SendGrid