Performance: Getting rid of temporary internal StringWriter

[mkarg]

TL;DR: This PR internally optimizes the JAX-RS API implementation code by getting rid of StringWriter for purely temporary use case.

Requesting fast-track according to our Committer Conventions, as this PR is a non-API, non-spec, non-javadoc change.

For those interested in the details:

• StringWriter is implicitly synchronized due to its internal use of StringBuffer, while StringBuilder is non-synchronized. Using synchronized code in single-threaded contexts is not needed.
• Providing initial content to the constructor reduces number of interally copied bytes on later append thanks to bigger upfront buffer allocation.
• Appending null automatically appends "null". This is implied by StringBuilder, no need for custom code.
• Appending a single char is faster than adding a single-character String.
• StringBuilder has fluent API; no need to repeat sb again and again.
• Since long time (JDK 8 and earlier), String + operation implies creation of StringBuilder, and is proven to be always faster than an explicit StringBuilder (see Video "String Concatenation - JEP Café #7" for the benchmark results).
• Since JEP 280 (JDK 9), String + operator is indified, leading to higher performance in future, just by upgrading JRE.