The getContentAsString method was originally added in d9b8826 to avoid
the extra copying inherent to calling ByteArrayOutputStream.toByteArray;
however, in f83c609 the class was updated to instead use
FastByteArrayOutputStream, and in the process the extra copy was brought
back when getContentAsString was changed to call toByteArray.
Switch to calling toByteArrayUnsafe, a method provided by
FastByteArrayOutputStream, which avoids the extra copy; since we
immediately pass the byte array to the String constructor, and it isn't
accessed anywhere else, the usage is safe.
See gh-31731
e452c2e89c