test: outputSpec: rework 'is undefined when httpinvoke results in an error' into 'is undefined when server does not send anything'
test: outputSpec: add 'is undefined when response is empty bytearray' and 'is undefined when response is empty string'
dummyserver.js: handle /text/utf8/empty GET and /bytearray/empty GET
-browser: do not use responseType=json, rework getOutput, getOutputLength