Fix fetch_formatter api_format

[ericproulx]

Fix #2171

This PR also introduce the function api_format which can be used in route. Instead of env['api.format'], we can now user api_format :format. Seems better than exposing internal api.format.

I've also took the opportunity to set HTTP_VERSION in Grape::Http::Headers and use it. Finally, instead of using [] || pattern for a Hash, I've changed it to .fetch() {} which seems a bit faster.

irb(main):002* Benchmark.ips do |x|
irb(main):003*   x.report("fetch") { {}.fetch(:non_existance, true) }
irb(main):004*    x.report("[] ||") { {}[:non_existance] || true }
irb(main):005> end; nil
ruby 3.3.4 (2024-07-09 revision be1089c8ec) [arm64-darwin23]
Warming up --------------------------------------
               fetch     1.511M i/100ms
               [] ||     1.446M i/100ms
Calculating -------------------------------------
               fetch     15.149M (± 3.1%) i/s   (66.01 ns/i) -     77.071M in   5.093208s
               [] ||     14.418M (± 1.1%) i/s   (69.36 ns/i) -     72.310M in   5.015870s
=> nil

[dblock]

Some text nits. Thanks.