Improve XML compatibility with trace-level logging

Some providers, notably Azure, include a byte-order mark in their XML
responses.  ParseSax.apply buffers these responses in a String when
users enable trace-level logging to include the response in any thrown
exceptions.  InputSource(InputStream) skips these byte-order marks
while InputSource(Reader) does not, yielding a SAXParseException.
Add a comment to the code to explain why we're not using `from` here...roughly the same comment as in the PR description, I guess?