INT-3427: Add `JsonNodeToStringConverter`

JIRA: https://jira.spring.io/browse/INT-3427
We just haven't had a test-case with `<int:converter>` here before. The `File` resolution for expression just doesn't work with the result of `CustomConversionServiceFactoryBean`. Revert it and you'll see:
```
org.springframework.expression.spel.SpelEvaluationException: EL1001E:(pos 0): Type conversion problem, cannot convert from java.lang.String to java.io.File
```
Because of : 
```
((GenericConversionService) service).removeConvertible(Object.class, Object.class);
```
in the `CustomConversionServiceFactoryBean`