Getting error `json_oauth1_signature_mismatch` when calling methods with data

[kristofdidrickson]

Hi, I'm just try to post some data using an authenticated request. Nothing crazy or special. Any calls that don't involve data work fine, but when I try to pass a data object to the same Wordpress OAuth instance, I get this error:

{ code: 'json_oauth1_signature_mismatch',
  message: 'OAuth signature does not match',
  data: { status: 401 } }

Which would suggest there's a bug in generating the signature when there's data involved.