JSON payloads are sometimes prepended with a static string prefix
to prevent Cross Site Scripting Inclusion attacks (XSSI).
Prior to this commit, doing so would fail the MockMvc
`JsonPathResultMatchers` since they're considering the whole response as
the JSON payload.
This commit adds a new `JsonPathResultMatchers.prefix` method that
configures the matchers to check for the presence of that string (i.e.
fail if it's not there) and only consider the rest of the response body
as the JSON payload for other assertions.
Issue: SPR-13577
7ae44c2565