Replace Hash#merge with Utils#deep_merge for connection options

[xkwd]

Description

This is an attempt to close the #1248 issue by following the proposed solution #1 in this comment.

Prior to this change:

Faraday.default_connection_options = { headers: { user_agent: 'My Agent 1.2' } }
Faraday.new('http://example.co', headers: { 'CustomHeader' => 'CustomValue' }).headers 
# => { 'CustomHeader' => 'CustomValue', 'User-Agent' => 'Faraday v2.0.0.alpha-2' }
Faraday.new(url: 'http://example.co', headers: { 'CustomHeader' => 'CustomValue' }).headers 
# => { 'CustomHeader' => 'CustomValue', 'User-Agent' => 'Faraday v2.0.0.alpha-2' }

After this change:

Faraday.default_connection_options = { headers: { user_agent: 'My Agent 1.2' } }
Faraday.new('http://example.co', headers: { 'CustomHeader' => 'CustomValue' }).headers 
# => { 'CustomHeader' => 'CustomValue', 'User-Agent' => 'My Agent 1.2' }
Faraday.new(url: 'http://example.co', headers: { 'CustomHeader' => 'CustomValue' }).headers 
# => { 'CustomHeader' => 'CustomValue', 'User-Agent' => 'My Agent 1.2' }

The only issue with the proposed solution was that the deep_merge from Faraday::Utils didn't work out of the box for an Options Struct like RequestOptions and SSLOptions from Faraday::ConnectionOptions.

[iMacTia]

Thank you @xkwd, this change is very much welcome!
The biggest issue I had with this solution, as I mentioned in my comment on the issue, was that this is effectively backwards-incompatible and might cause surprise for users.

Good news though is that we're in alpha testing for Faraday 2.0 🎉! So I'm happy to merge this is in and simply drop a comment on the UPGRADING.md guide 🙌

The only downside is that if you need this in one of your projects, you'll need to upgrade to Faraday 2.0

[iMacTia]

I just realised there's no rollback of the original value after these test have run 😄.
I'll be fixing this on the main branch, so no worries at all, but please keep this in mind next time you write tests for a global configuration 👍