RestTemplateBuilder auto detecting netty results in broken ssl
Hi,

I was playing around with `RestTemplateBuilder` and it's `RequestFactory` auto detection.
While doing this I noticed that for netty, the resulting `Netty4ClientHttpRequestFactory` will be misconfigured, resulting in ssl/https calls being broken with such a client.

The reason is that `Netty4ClientHttpRequestFactory` sets up it's sslContext in an `afterPropertiesSet` method, which the `RestTemplateBuilder` never calls.

Declaring `Netty4ClientHttpRequestFactory` as a bean an setting it on the builder via `requestFactory(ClientHttpRequestFactory requestFactory)` will result in a working RestTemplate.

Affected versions:
* Verified on Spring boot 1.5.4
* Looking at the code, seems to be still in 2.0.0

Steps to reproduce:
* Have spring-boot-starter-web and netty-all on the classpath.
* Construct a Resttemplate via `new RestTemplateBuilder().build()`
* Try to access an HTTPS Url

Result:
```
2017-07-19 23:30:00.864  WARN 11639 --- [ntLoopGroup-2-1] io.netty.channel.ChannelInitializer      : Failed to initialize a channel. Closing: [id: 0x98b62de7]

java.lang.IllegalArgumentException: sslContext should not be null
```