* GH-110: Fix filter function for emitting `null`
Fixes https://github.com/spring-cloud/stream-applications/issues/110
Turns out when functions are composed in the Spring Cloud Stream environment,
they are called via reactive wrappers which don't allow to emit `null` from the
`map()` operator.
* Make `filterFunction` fully reactive to reply on the `Flux.filter()` operator
* Add `config-common` for auto-conversion string configuration options into `Expression` instances
* Add `spring-boot-starter-json` dependency since it is required by the `SpelExpressionConverterConfiguration`
* Add `reactor-test` dependency to test the final solution
* Remove redundant dependencies from the `filter-processor`
* * Add `proxyBeanMethods = false` into `FilterFunctionConfiguration`
* Fix default expression to `true` instead of `payload`, which does not fit to filter logic
* Fix JavaDoc for `expression` property
* Remove redundant `application.properties` from the `filter-function`
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