Update CHANGES.md

Added an issue that was fixes in 0.3.15 but was not present in the release notes.
Fixes #269 (`ServerSentEventDecoder` stops decoding after receiving an illegal field)

Now ignoring all data till an EOL after receiving an unidentified field, in essence ignoring that event.
Fixes for 0.3.18

#269(`ServerSentEventDecoder` stops decoding after receiving an illegal field): Now ignoring all data till an EOL after receiving an unidentified field, in essence ignoring that event.
#270: HttpClient should only do relative redirects
#271: URI fragment must be inherited by the redirect location
#272(Guaranteeing sequential notifications from operators/Subjects): Using `SerializedSubject` in `ObservableConnection.getInput()` and `SerializedObserver` in `UnicastContentSubject`
#257 Support relative HTTP redirects
Merge pull request #273 from NiteshKant/0.x

Fixes for 0.3.18
Updated CHANGES.md for 0.3.18
Updated CHANGES.md for 0.3.18
Merge pull request #274 from NiteshKant/0.x

Updated CHANGES.md for 0.3.18