added toString() method for CaseInsensitiveHeaders
added unit test for the class

Signed-off-by: Alex <alexlehm@gmail.com>
added toString() method for CaseInsensitiveHeaders (copied from 2.x branch)
added unit test for the class (cleaned up unit tests and fixed checkstyle errors)

Signed-off-by: Alex <alexlehm@gmail.com>
Merge pull request #940 from alexlehm/bz430965

added toString() method for CaseInsensitiveHeaders