Fix for "Transfer-Encoding" header occasionally returned in lower-case

Relatively rarely, connecting to the Streaming API at https://stream.twitter.com/1.1/statuses/filter returns the "Transfer-Encoding" header in lower-case: "transfer-encoding: chunked". This currently triggers the Exception "Twitter did not send a chunking header. Is this really HTTP/1.1? (...)" (line 745).

I've opened a discussion on the Twitter dev site for this bug (currently awaiting moderation): https://dev.twitter.com/discussions/27837

Whether they can resolve it or not, it doesn't hurt to tolerate the lower-case version.