450855 GzipFilter MIGHT_COMPRESS exception

Disabled tests that tried to apply the GzipFilter to default async content generation.  These
tests will never work as GzipFilter wraps the response and the default async does not preserve wrappers.

For the MIGHT_COMPRESS issue, improved the filter to handle the case that it has already been applied to
a request/response and that the state is already mightCompress.
450873 Disable tests that downcaste wrapped GzipFilterResponses
450855 GzipFilter MIGHT_COMPRESS exception

Re-enabled some default GzipFilter tests.  These will work because the filter rewraps the response on
the dispatch.  It is only the timeout test that will not work with default.
Merge remote-tracking branch 'origin/jetty-9.2.x'

Conflicts:
	jetty-server/src/main/java/org/eclipse/jetty/server/handler/gzip/GzipHttpOutputInterceptor.java
	jetty-servlets/src/main/java/org/eclipse/jetty/servlets/AsyncGzipFilter.java
	jetty-servlets/src/test/java/org/eclipse/jetty/server/handler/gzip/GzipContentLengthTest.java

Added Interceptor chain so that double gzip interception can be prevented.