title: @FeignClient from openfeign and Observable leads to fasterxml.jackson.databind.exc.InvalidDefinitionException
body: <p>I've a project with the following feign client</p>
  
  <pre><code>@FeignClient(name = "name", url = "${tokenService}")
  public interface TokenService {
      @RequestMapping(path = "/SLJsonWebTokenWrapper/token/wrap", method = RequestMethod.POST,
          consumes = MediaType.APPLICATION_JSON_VALUE, produces = MediaType.APPLICATION_JSON_VALUE)
      Observable&lt;Token&gt; getToken(@RequestHeader("externalSystemCode") String systemCode,
                             @RequestBody TokenRequest tokenRequest);
  }
  </code></pre>
  
  <p>and the following dependency</p>
  
  <pre><code>org.springframework.cloud:spring-cloud-starter-feign
  </code></pre>
  
  <p>Everyting works fine.
  When I update my project to Spring Boot 2 (and use <code>org.springframework.cloud:spring-cloud-starter-openfeign</code>) requests to this service fail with exception</p>
  
  <pre><code>c.f.j.d.e.InvalidDefinitionException: Cannot construct instance of 
  `rx.Observable` (no Creators, like default construct, exist): cannot 
  deserialize from Object value (no delegate- or property-based Creator)
   at [Source: (ByteArrayInputStream); line: 1, column: 2]
  at c.f.j.d.e.InvalidDefinitionException.from(InvalidDefinitionException.java:67)
  at c.f.j.d.DeserializationContext.reportBadDefinition(DeserializationContext.java:1452)
  at c.f.j.d.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1028)
  at c.f.j.d.d.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1297)
  at c.f.j.d.d.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:326)
  at c.f.j.d.d.BeanDeserializer.deserialize(BeanDeserializer.java:159)
  at c.f.j.d.ObjectMapper._readMapAndClose(ObjectMapper.java:4013)
  at c.f.j.d.ObjectMapper.readValue(ObjectMapper.java:3084)
  ... 1 frames excluded
  ... 99 common frames omitted
  Wrapped by: o.s.h.c.HttpMessageConversionException: Type definition 
  error: [simple type, class rx.Observable]; nested exception is 
  com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot 
  construct instance of `rx.Observable` (no Creators, like default 
  construct, exist): cannot deserialize from Object value (no delegate- 
  or property-based Creator)
  at [Source: (ByteArrayInputStream); line: 1, column: 2]
  at o.s.h.c.j.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:240)
  at o.s.h.c.j.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:225)
  ... 3 frames excluded
  at f.o.OptionalDecoder.decode(OptionalDecoder.java:36)
  at f.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:170)
  ... 93 common frames omitted
  Wrapped by: f.c.DecodeException: Type definition error: [simple type, class rx.Observable]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `rx.Observable` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
   at [Source: (ByteArrayInputStream); line: 1, column: 2]
  </code></pre>
  
  <p>How can I fix it?</p>
  
  <p>P.S. This question is similar to <a href="https://stackoverflow.com/questions/42320242/spring-cloud-netflix-and-hystrixobservable-jsonmappingexception">Spring cloud netflix and HystrixObservable --&gt; JsonMappingException</a>, that has no answer though</p>