1.0.0-b6

* Use SFINAE on return values
* Use beast::error_code instead of nested types
* Tidy up use of GENERATING_DOCS
* Remove obsolete RFC2616 functions
* Add HTTP field value parser containers:
    - ext_list
    - param_list
    - token_list

API Changes:

* ci_equal is moved to beast::http namespace, in rfc7230.hpp

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