private void bindVariables(JsonNode actualJson, RestTest restTest) {
        for (String headerName : restTest.getExpectedResponseHeaders().keySet()) {
            if (headerName.startsWith("X-RestTest-BindTo-")) {
                String bindExpression = restTest.getExpectedResponseHeaders().get(headerName);
                String bindVarName = headerName.substring("X-RestTest-BindTo-".length());
                String bindValue = evaluate(bindExpression, actualJson);
                log("-- Binding value in response --");
                log("\tExpression: " + bindExpression);
                log("\t    To Var: " + bindVarName);
                log("\t New Value: " + bindValue);
                if (bindValue == null) {
                    System.clearProperty(bindVarName);
                } else {
                    System.setProperty(bindVarName, bindValue);
                }
            }
        }
    }
Binds any variables found in the response JSON to system properties so they can be used
in later rest tests.

@param actualJson
@param restTest
test/common/src/main/java/io/apiman/test/common/util/TestPlanRunner.java