This commit introduces a toString() overload in
FastByteArrayOutputStream that accepts a Charset in order to mirror the
method that was introduced in ByteArrayOutputStream in JDK 10,
including a special case for when a single buffer is in use internally
to avoid the need to resize.
This commit also updates getContentAsString() in
ContentCachingRequestWrapper to use this new toString(Charset) method.
Closes gh-31737
7cdacf3083