(PE-6019) Set request body character encoding to UTF-8

This commit sets the character encoding for the body of a request to
UTF-8.  Previous code did not set a character encoding and so a default
of ISO-8859-1 was implicitly used.  This change assumes that UTF-8 is a
more reasonable default for clients although work should be done later
to make this externally configurable.
No.  Unlike the other constructor, this one doesn't declare that it can throw an `UnsupportedEncodingException`.  Looking at the underlying implementation, the `UnsupportedEncodingException` is only thrown when the input arguments cannot be coerced into a `Charset` instance.  This is unambiguous with the new code here because the `Charset` instance is given explicitly as a parameter, `Consts.UTF_8`.