Null Check added for 'value' before doing value.getClass() in getAllHeaders()

Null Check added for 'value' before doing value.getClass() in getAllHeaders() - lines 237-239 and 255

FIX for the issue: http://jira.grails.org/browse/GPCACHE-37
Merge pull request #5 from techfoobar/master

Null Check added for 'value' before doing value.getClass() in getAllHead...