Dropwizard/plaintext resource now returning precomputed byte array

This could be considered as a performance optimisation. It is similar to the plaintext implementation with [undertow/undertow-edge](https://github.com/TechEmpower/FrameworkBenchmarks/blob/master/frameworks/Java/undertow-edge/src/main/java/hello/PlaintextHandler.java). Obviously without the NIO optimisation.