use VALID_REQUEST_KEYS constant for Excon if defined

Addresses https://github.com/bblimke/webmock/issues/403

While digging further into this, I was able to track down the introduction of the constant to release 0.27.0:
https://github.com/excon/excon/commit/a93c591762628169f94a0bed04c1aab9e629ede1

So I thought it'd be nice to just get rid of that ` if Excon::VERSION >= '0.27.5'` and replace with a constant check.  Also added a spec to make sure it's not broken.

Ran specs locally and everything looks green.