Add support for $browser.addHeader() when running Synthetics locally

[excalbian]

Since I'm using the $browser.addHeader() function in my Synthetics, they currently fail when trying to run locally with an error similar to the example below.

$browser.addHeader("x-dynaTrace", "NA=SAMSimple;SI=NewRelicSynthetics");
         ^

TypeError: $browser.addHeader is not a function
    at Object.<anonymous> (/synthetics/samProd2Dev.js:53:10)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.runMain (module.js:604:10)
    at run (bootstrap_node.js:394:7)
    at startup (bootstrap_node.js:149:9)
    at bootstrap_node.js:509:3