Issue #10131 - Test case showing ERROR param merging works as-is.

[joakime]

Turns out the existing HEAD of jetty-12.0.x does perform parameter merging with ERROR dispatch.

Adding testcase to validate this.

[gregw]

So we are correctly merging the parameters for both forward and error dispatches. The only problem is that for both we are not creating a merged query string.

[joakime]

So we are correctly merging the parameters for both forward and error dispatches. The only problem is that for both we are not creating a merged query string.

See past discussion on the topic of merging of query strings at #2074 (there's a lot of messages to go through, from multiple people, but it's an interesting read on how we've been interpreting the vague servlet spec in this regard)