r/pagerduty_user: set invintation_sent

[gordonbondon]

While working on #126 and running acceptance tests with new SDK I discovered this bug:

make testacc TEST=./pagerduty TESTARGS='-run=TestAccPagerDutyUser_import'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./pagerduty -v -run=TestAccPagerDutyUser_import -timeout 120m
=== RUN   TestAccPagerDutyUser_import
--- FAIL: TestAccPagerDutyUser_import (10.70s)
    testing.go:568: Step 1 error: ImportStateVerify attributes not equivalent. Difference is shown below. Top is actual, bottom is expected.

        (map[string]string) {
        }


        (map[string]string) (len=1) {
         (string) (len=15) "invitation_sent": (string) (len=5) "false"
        }

[heimweh]

Thank you so much for this @gordonbondon! 🙂 This LGTM 👍