Better warning message for missing Box custom methods

[ppaulojr]

We could modify the Assertion code to handle better cases where the developer forgot to write the code to handle the custom Box.

    SEL mappingSelector = NSSelectorFromString([NSString stringWithFormat:@"boxValueAs%@:", mappingType]);
    AssertTrueOrReturnNoBlock([self respondsToSelector:mappingSelector], ^(NSError *error) {
    });

In the AssertTrueOrReturnNoBlock we could print the expected selector.

[krzysztofzablocki]

sounds like a good improvement, would you be able to submit a PR for that?