Validate TE header is legal for HTTP2.

As per draft-16 section 8.1.2.2:
The only exception to this is the TE header field, which MAY be
present in an HTTP/2 request; when it is, it MUST NOT contain any
value other than "trailers".

https://tools.ietf.org/html/draft-ietf-httpbis-http2-16
	Change on 2014/12/01 by ctiller <ctiller@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81111980