(PE-6019) Encode string request body per Content-Type header

This commit encodes a request body string per the content of the
Content-Type header.  In the event that no charset is specified in the
Content-Type header, the charset is set to UTF-8.  Previously, the
request body string was always encoded to ISO-8859-1 and was not
necessarily in sync with the value of the Content-Type header.
Is state involved here? Shouldn't we be returning the result of the `assoc` above?