Fix rescue all header versioning regression

[suan]

A suggested fix for #968

There are two failing specs which show that unfortunately there is no way to rescue_from an invalid version, since it's possibly valid and needs to propogate. This was also the case before the regression was introduced.

[dblock]

The code looks good, but I am confused to what the spec failures mean - are the specs wrong? If so they need to change.

Also please update CHANGELOG and squash the commits.

[dblock]

I am curious what @dm1try thinks about this.

Also, this definitely needs to be documented in both README and the behavior change in UPGRADING.

Finally, maybe we need to make this behavior configurable?

[suan]

@dblock The spec failures show that with my changes it's (unfortunately) not possible to rescue any Grape::Exceptions::InvalidHeaderVersions - as we'll always need to pass those through to allow multi-versioned endpoints to work. (If there's some other way that you know of, that'd be more ideal)

[croeck]

The changes seem good to me. All failing tests are invalid and did not regard the scenario as defined in the related issue.

[dblock]

@suan remove the bad tests, squash the commits please, update README, UPGRADING and CHANGELOG.

[suan]

@dblock I've made the suggested changes - let me know if there's anything you'd need, thanks!

[dblock]

This is good, merging.

[suan]

Awesome, appreciate it @dblock!