Does not set accept-encoding header when zuul proxies request if already present.  Fixes #2998 (#3011)

This fixes problems in testing while maintaining backwards compatibility.

Allows upgrade of zuul to 1.3.1

This reverts commit 61a920e0.

Fixes #2979

Closes #2975

* Fixed: remove misplaced StringEncoder setting place when dealing with protobuf

* test SpringEncoder when protobuf is not in classpath

* Fixed content length handling issues. (#2894)

* Added test setup and cleanup for RequestContext. (#2894)

Fix #2910

Avoid problems if OKHTTP is not on the classpath.  Fixes #2797 

* Retryable Clients should validate the service server and throw an exception if necessary

* fixing spelling error

* RetryableRibbonLoadBalancingClient should throw a ClientException which can be handled by the RibbonRoutingFilter

* moving service instance validation and service instance chooser to the AbstractLoadBalancingClient

* Updating RetryCallback within RetryableRibbonLoadBalancingHttpClient to handel Exception