when If-Range doesn't match, ignore Range

[royfielding]

part of #914 Section 13.2.2

    When the method is GET and both Range and If-Range are present,
    evaluate the If-Range precondition:
        if the validator matches and the Range specification is
        applicable to the selected representation, respond 206
        (Partial Content)

    Otherwise,
        all conditions are met, so perform the requested action and
        respond according to its success or failure.

I think that if the If-Range doesn't match, we're supposed to ignore the
Range header field when performing the requested action, which doesn't
seem to match up with this unadorned directive to "perform the requested
action" (which would include the Range header field).
(We might also change point (5) to use the "if true" phrasing that the
other items use in the context of evaluating the precondition.)