This commit builds on the changes made in the previous commit to
make wider use of the new getContentAsString methods on
OperationRequest and OperationResponse. It also adds such a method to
OperationRequestPart. Any code that previously got the content as a
byte array and then created a String has been updated to either use
getContentAsString or to use the charset from the Content-Type header
when creating a String.
See gh-126
3f6d4b9639