fixed the failing test browsermob-core/src/test/groovy/net/lightbody/bmp/proxy/NewHarTest.groovy testCaptureResponseCookiesInHar()

[kazurayam]

This PR is targeted to the current master branch of 2.1.6-SNAPSHOT version.

The test browsermob-core/src/test/groovy/net/lightbody/bmp/proxy/NewHarTest.groovy failed. The failure blocked me to accomplish mvn test and mvn install. I checked the source of browsermob-core/src/test/groovy/net/lightbody/bmp/proxy/NewHarTest.groovy class,　method testCaptureResponseCookiesInHar(). I found a small problem to be fixed. See the issue in the forked repository in my GitHub accout for what I have done in detail:

• browsermob-core/src/test/groovy/net/lightbody/bmp/proxy/NewHarTest.groovy　testCaptureResponseCookiesInHar() failed kazurayam/browsermob-proxy#3

By my fix, mvn test passes, mvn install succeeds.