Add security verifications.

Add verification about:
    * the event format (UTF-8, all except :, \n and \r);
    * if the client accepts the text/event-stream type. If no, a 406
      status code is sent back and an exception is thrown.
[auto] Merge from Eventsource

    • #2632c47 — Add security verifications. (Ivan Enderlin, 2013-04-10)

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Merge remote branch 'library-eventsource/master'