Java DSL: Assertion error "headerExpressions must not be empty"

Fix for spring-projects#101

If we don't add a `headerExpression` on the `EnricherSpec` class (because we simply only want to enrich the payload), we will get an assertion error in the `ContentEnricher` class saying 
```
headerExpressions must not be empty
```

The is also the same when we only want to use header expressions.


I have signed and agree to the terms of the SpringSource Individual Contributor License Agreement.