Work around the lack of response headers

When Streamy is handling an error response, the response body may be
in JDON format, but there's currently no access to the Content-Type
header to check. So, just try to parse it to find out.
Do the best with error bodies in the face of the unknown

Some HTTP back ends may not return the response body type. In that
case, try parsing the response as JSON, in case it is. If the body
type is available, then parse the body as JSON when it is, in fact,
JSON.
Merge pull request #53 from gsadams/master

Work around the lack of response headers
Honor return value of retryFuture.
Make retryImmediately honor retry strategy signature.
Merge remote-tracking branch 'upstream/master' into exception-and-retry
ProxyException -> StreamyRpcException
Adding first test cases
RetryingRequestHandler working.
Delete ProxyException.