Spring Avoid byte array copy in getContentAsString

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.