#277 fixing faulty match check in built-in methods on return/target type...

....

The check was carried out in the wrong direction. Iso carrying out if a return type is assignable to a target type, MapStruct checked the other way around (target type assignable to return type). 

In fixing, this poses a problem for a typed variable, e.g. `<T> T jaxbElemToValue( JAXBElement<T> element )`. The erasure of the return type is an Object, which is never assignable to, lets say T being a String.

Therefore to additional checks have been added on the assumption that a a Typed variable erasure is an java.lang.Object. 

So the full check now is:
```java
        if ( getReturnType().erasure().isAssignableTo( targetType.erasure() )
            && sourceType.erasure().isAssignableTo( getParameter().getType().erasure() ) ) {
            return doTypeVarsMatch( sourceType, targetType );
        }
        if ( getReturnType().erasure().getFullyQualifiedName().equals( "java.lang.Object" )
            && sourceType.erasure().isAssignableTo( getParameter().getType().erasure() ) ) {
            // return type could be a type parameter T
            return doTypeVarsMatch( sourceType, targetType );
        }
        if ( getReturnType().erasure().isAssignableTo( targetType.erasure() )
            &&  getParameter().getType().erasure().getFullyQualifiedName().equals( "java.lang.Object" ) ) {
            // parameter type could be a type parameter T
            return doTypeVarsMatch( sourceType, targetType );
        }
        return false;
```
Perhaps there are more elegant ways to achieve this than a String equals check on the FQN.