Fix getting content-length from response header

[aditya0711]

If the _contentLength attribute is empty, domino affect on other components is observed - such as Last Errors and Errors and other tabs..

To fix.. check if _contentLength is not empty, if it is, check the headers

[coveralls]

Coverage decreased (-0.3%) to 89.718% when pulling de6c59a on aditya0711:core-stats-response-clength-#42 into c7c3e6b on slanatech:master.