WebMock::Util::Headers.normalize_headers modifies underscores in headers

[bmo]

Related: #863

Headers like SYNCHRONOUS_ONLY are normalized to Synchronous-Only, which will break any tests verifying an underscore in a header (there are existing APIs that expect underscores in certain headers, commercial example: https://developer.blackhawknetwork.com/hawkmarketplace/docs/placing-egift-real-time-orders).

WebMock::Util::Headers.normalize_headers({'SYNCHRONOUS_ONLY'=>'true'})
=> {"Synchronous-Only"=>"true"}

I note that other libraries intentionally or inadvertently provide a means of bypassing this behavior. For example, the key that is being normalized could be checked for string-like characteristics, and normalization skipped if so.

The Faraday gem, also subject to an MIT license, handles it this way: https://github.com/lostisland/faraday/blob/d0efa092829362f95d4b71d3bcd3c52189b74277/lib/faraday/utils/headers.rb#L41
By using the hash-rocket syntax to define a header (e.g. 'SYNCHRONOUS_ONLY' => 'true') the header name could pass unmolested.

[bmo]

Sorry, with better looking, I now note that this is a duplicate of #474