use HTTP 302 status for UI redirect

[d3ns0n]

@bnasslahsen

here is the PR for switching to 302 instead of 307 when using webflux-ui. This PR also contains some cleanups for affected test classes:

• remove never thrown Exceptions from throws list
• remove IntelliJ warning Argument 'getResult.getResponseBody()' might be null by asserting to not null
• switch from org.junit.jupiter.api.Assertions.assertTrue to org.assertj.core.api.Assertions.assertThat for better assertion error output

[bnasslahsen]

@d3ns0n,

Great PR. This one had more impacts on tests !
Thank for your help.
We should release the new version v1.5.6, before the end of the week, which will ship your changes as well.