Allow for @HystrixCommand to be used on parameterized return type

The current implementation of hystrix-javanica doesn't work with methods that have a parameterized method return type. I.e. the following (somewhat silly example) would not work:
```java
@HystrixCommand
public <T> T echo(T value) {
    return value;
}
```
This is because the code that determines the execution type from the method return type incorrectly matches `Object` to `Future` and then marks it for asynchronous execution. This leads to an exception at runtime. E.g.:
```java
final String answer = echo("hello");
```
would cause a class cast exception.

Correcting the logic for determining the execution type fixes this issue.