fix: don't JSON.stringify body if its already a string

[individual-it]

if the body is already a string don't try to stringify it

fixes #577

alternatively we could get rid of JSON.stringify all together, the limited e2e tests also pass then

[individual-it]

I don't see why CI would fail, but it must be because of the change #579 passes
digging into it

[individual-it]

or it is a timeout issue, here is passed in around 8sec

so 10 sec timeout might be just to little

[mefellows]

Thanks, this looks ok to my eyes.

The failure with the Windows build is a know one (there is an issue with message pact on Windows due to argument spawning).

I'll look at bringing it in tomorrow.

[TimothyJones]

Thanks very much for this! One small comment about side effects, other than that this is good to go.

[TimothyJones]

Merged! Thanks again!