Unit Test on RequestId Middleware to validate requests with a X-Request-ID header

[pafuent]

Add a new Unit Test to request_id_test.go to check that the middleware doesn't change a preexisting X-Request-ID header on the request.
Basically is a test that exercises the false path of this code

echo/middleware/request_id.go

Lines 51 to 54 in 6caec30

	rid := req.Header.Get(echo.HeaderXRequestID)
	if rid == "" {
	rid = config.Generator()
	}

[iambenkay]

I'll work on this and send in a PR