Fix default decoder's support for Response decoding, clean up usage of S...

...tringDecoder

Decoder.Default's decoding to Response didn't actually work; the reader would always
be closed when used from Feign, as it depended on the url connection, which would
have been closed by the time the Response object was returned to the client.

This wasn't noticed because the default decoder tests don't use the mock web server.
There will be test coverage added for this shortly as part of the enhancements to
support a Builder.