Do not convert already-string bodies to string

...doing so adds extra double-quotes around original string.

Running mocha the tests pass (although i haven't dug into them).

I was unable to POST with Connection.prototype.apexRest to the <pre>https://instance.salesforce.com/services/apexrest/v.9/members</pre> the required parameters in the format it was expected because of the double-quote issue.