Allow DefaultHttp2Headers to be forced to lowercase.

Motivation:
I came across an issue when I was adding/setting headers and mistakenly
used an upper case header name. When using the http2 example that ships
with Netty this was not an issue. But when working with a browser that
supports http2, in my case I was using Firefox Nightly, I'm guessing
that it interprets the response as invalid in accordance with the
specifiction
https://tools.ietf.org/html/draft-ietf-httpbis-http2-14#section-8.1.2

"However, header field names MUST be converted to lowercase prior
to their encoding in HTTP/2.  A request or response containing
uppercase header field names MUST be treated as malformed"

This PR suggests converting to lowercase to be the default.

Modifications:
Added a no-args constructor that defaults to forcing the key/name to
lowercase, and providing a second constructor to override this behaviour
if desired.

Result:
It is now possible to specify a header like this:
Http2Headers headers = new DefaultHttp2Headers(true)
    .status(new AsciiString("200"))
    .set(new AsciiString("Testing-Uppercase"), new AsciiString("some value"));

And the header written to the client will then become:
testing-uppercase:"some value"
@Scottmitch The no-args constructor could just as well be ```super(true)```, I delegated to the other constructor for readability. Is that what you meant, or were you asking about the second constructor? The idea with that one would be to give user the option to override the default.