Use XML body format instead of form encoding

[bartonip]

This wrapper gets a 500 error when submitting POST requests to Payroll API because it submits the body of the request as an XML string in the application/x-www-form-encoded format.

As outlined by Xero support below, the Payroll API does not support this format and requires XML or JSON. Since we are converting dictionaries to XML anyway, we may as well update save_or_put to submit the full XML in the body of the request and supply an application/xml header.

[freakboy3742]

Looks reasonable; surprising that this hasn't come up previously.

However, we need a test to verify that this behavior is being applied (and will stay that way in future).

[bartonip]

@freakboy3742 tests added as requested

[freakboy3742]

Looks good - thanks for the patch!