added toString() method for CaseInsensitiveMultiMap (version for 2.x)

pull request for the change in 2.x branch, I have left out the unit test for this one.


I used the toString() method initially, but that uses a = sign as separator, I then preferred to have the output look like the header format, e.g.

    Header: header1
    Header: header2

so I used this format