Sigpipe transport

Put unit test helper functions in internal package to make them available to unit tests of all packages

NewHTTPTransport now sets SO_NOSIGPIPE on iOS, in a doubtful attempt to prevent SIGPIPE crashes (as golang/go#17393 mentions this is already fixed in gomobile).

When the identity provider does not provide a validity of the credentials, the Java implementation takes now + 6 months while we took now + 52/2 weeks, which is not always the same. This commit takes the approach of the Java implementation.