Adds conditional on Ribbon class to RibbonAutoConfiguration

 Previously, when feign used the Ribbon RestClient, an exception was thrown if Content-Length was set. Over time, feign has moved away from RestClient, yet removing the header still happened.

 Fixes gh-1705

(cherry picked from commit 15e01dfe)

Makes Spring Retry optional for Zuul

Add Retry Logic To Zuul Using Spring Retry.  Fixes #1295

Fixes gh-1663

fixes gh-1616

A new version of httpclient introduced by boot changed the behaviour that previously allowed the charset. It now has to be parsed out and passed to the apache `ContentType` object explicitly.

fixes gh-1649

Set the default buffer size to 8192 to match Tomcat and Jetty defaults
Use a buffer per worker thread and use it for subsequent requests

This reverts commit f5c69208.

Use Spring Retry For Failed Requests Using Feign

Fixes gh-1546

No longer use RetryTemplate bean since the retry policy being used is dependent on the request.  We don’t want two Feign requests stomping on each other by switching out the retry policy for the template so each request now created a new template to use.

Use configuration from IConfigClient reference to always use the most up to date configuration

Uses configuration from IConfigClient reference to always use the most up to date configuration.  Fixes #1481.

Defaults were set too late and were overriding values set in the environment.

Specifically, spring.application.name took precedence over eureka.instance.appname.

 fixes gh-1516

Specifically, add `@ConditionalOnBean(EurekaClient.class)`.

Fixes an issue where eureka and ribbon are on the classpath, eureka.client.enabled=false which would cause a bean not found exception.

fixes gh-1511