#132 SipPort ignorableHeaders not set as intended

[Tom Smith]

SipPort defines ignorableHeaders, which is an unmodifiable Set backing another Set. However some Strings are added to the Set erroneously ("content-type" and "content-length"). In practice it probably does not matter much, since these get set implicitly as a side-effect of calling setContent(). Furthermore, it seems like overkill to use unmodifiable Sets in the first place.