fixed serialization problems for mime messages

[mmertens93]

Fixes microsoftgraph/msgraph-sdk-serviceissues#112

Changes proposed in this pull request

• When sending mime messages (content-type "text/plain"), the library should not enclose the request body with double quotes

Additional Information

• After applying this patch, we are now getting a 202 - Accepted from the Server when trying to send a mime message
• However, there seems to be a problem on the Server Side, see: https://docs.microsoft.com/en-us/answers/questions/501593/sending-mime-via-graph-results-in-mangled-e-mail.html

[baywet]

Thanks a lot for the contribution! LGTM. @ramsessanchez would you mind doing some additional testing please?

[baywet]

ah one thing that'd be missing is a unit test for that, sorry I forgot it on the first review.

[ghost]

All CLA requirements met.

[baywet]

Thank you so much for adding the unit test @mmertens93 !
I did push a few additional commits to fix a linting issue on the unit test as well as address a missing javadoc comment from #249.
@ramsessanchez please have a look and merge at your earliest convenience.