title: Implicit Conversion of 'BOOL'(aka 'bool') to 'id' is disallowed by ARC
body: <p>I am trying to transform a value of Pending/Completed to a boolean True/False value</p>
  
  <p>Here is the code for that:</p>
  
  <pre><code>RKValueTransformer *transformer = [RKBlockValueTransformer valueTransformerWithValidationBlock:^BOOL(__unsafe_unretained Class sourceClass, __unsafe_unretained Class destinationClass) {
          return ([sourceClass isSubclassOfClass:[NSNumber class]]);
      } transformationBlock:^BOOL(NSNumber *inputValue, __autoreleasing id *outputValue, __unsafe_unretained Class outputClass, NSError *__autoreleasing *error) {
          // validate the input
          RKValueTransformerTestInputValueIsKindOfClass(inputValue, [NSNumber class], error);
          if([inputValue isEqualToNumber:@(Completed)]) {
              *outputValue = YES;
          } else if([inputValue isEqualToNumber:@(Pending)]){
              *outputValue = FALSE;
          }
          return YES;
      }];
  </code></pre>
  
  <p>However, I get the error: <code>Implicit Conversion of 'BOOL'(aka 'bool') to 'id' is disallowed by ARC</code></p>
  
  <p>When i try to set the outputValue to be YES...</p>
  
  <p>What's going on here?</p>
  
  <p>It should match this output:</p>
  
  <pre><code>{ 
     “IsCompleted”: true (nullable),
     “Desc”: null (“new description” on edit) 
  }
  </code></pre>