add echoHandler=true to responseHeader, support echoParams=all

optionally return what handler was used in the response header. This patch also extends echoParams so that it supports 'all' and 'none'

It makes a small API change to the protected SolrCore.setResponseHeaderValues() – it now passes in the handler that was used.

Some URLs to check (but remember that the /debug/dump handler prints out its own 'params')

http://localhost:8983/solr/debug/dump
http://localhost:8983/solr/debug/dump?echoParams=all&param1=A
http://localhost:8983/solr/debug/dump?echoParams=explicit&param1=A
http://localhost:8983/solr/debug/dump?echoParams=false&param1=A

to keep things reasonable, i'm mapping:
echoParams=true >> explicit
echoParams=false >> none