Unset the used correlation id after the filter, and restore the original one

Fixes https://code.google.com/p/wro4j/issues/detail?id=824
add unit test proving that the problem is fixed
Merge branch 'issue824' into 1.7.x
reduce visibility of a method which is not required to be overriden by test
Merge branch 'issue824' into 1.7.x
properly cleanup Context for most of the unit tests
Merge branch 'issue824' into 1.7.x
Merge branch '1.7.x' into issue822