Issue title: model and error sections not getting generated
 Issue body: I am using swagger-maven-plugin and have annotated my api as described in the tutorial. I noticed that the model and errors subsection of the output json doc are not getting generated. 'RevokeAccountRequest' is my model which I have annotated properly.

Example annotation:

```
@PUT
@Path("/customer/{customerId}/revoke")
@Produces(MediaType.APPLICATION_JSON)
@ApiOperation(value = "Revoke a customer account", notes = "More notes about this method")
@ApiImplicitParams(@ApiImplicitParam(name = "body", value = "Revoke date", required = true, dataType = "RevokeAccountRequest", paramType = "body"))
@ApiResponses(value = {
  @ApiResponse(code = 400, message = "One or more required input parameter missing."),
  @ApiResponse(code = 401, message = "Authentication failure."),
  @ApiResponse(code = 403, message = "Authorization failure."),
  @ApiResponse(code = 404, message = "SecretId not found."),
  @ApiResponse(code = 500, message = "Server Error."),
})  
```

 Comments: 
 Comment 0: Same reason as issue #33 

{'source': 'GithubIssues', 'repo_name_with_owner': 'kongchen/swagger-maven-plugin', 'issue_id': 26658243, 'repo_stars': 729.0}