fixes gh-2309i

These things are now much better dealt with in Spring MVC and WebFlux

fixes gh-1946, gh-2220, gh-2221, gh-2222

fixes gh-1946, gh-2220, gh-2221, gh-2222

Modfied RibbonCommandContext to make request entity resettable. fixes gh-892

* Remove @EnableDiscoveryClient and @EnableEurekaClient as there are now optional.  Fixes #2241.

SpringMvcContract: Ignore RequestMapping headers with no value.

According to:

https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/bind/annotation/RequestMapping.html#headers--

the `headers` field can take several formats:

1. Require the header with a concrete value: `"My-Header=myValue"`
2. Require the header with any value: `"My-Header"`
3. Require the header is absent: `"!My-Header"`
4. Require the header with a negated value : `"My-Header!=myValue"`

This fixes `parseHeaders` for cases 2-4 (which previously threw a `StringIndexOutOfBoundsException`).

Fixes gh-1778.

* simplify org.springframework.cloud.netflix.zuul.filters.ProxyRequestHelper#setResponse

* Update ProxyRequestHelper.java

Fallback cause is propagated to fallback provider

Adjust Ribbon connect and read timeout values

Cause can now be propagated to fallback provider in order to respond accordingly to a exception type.
New interface is introduced in order not to break any existing falback providers.

Update rxjava version