Previously, the Faraday testing adapter compared the request body to the
stubbed body just by calling `#==` if the stubbed body is present.
Sometimes, I want to check the equality between request and stubbed body
in more advanced ways. For example, there is a case that I want to check
only the parts of the body are actually passed to Faraday instance like
this:
```ruby
stubs = Faraday::Adapter::Test::Stubs.new do |stub|
stub.post('/foo', '{"name:"YK","created_at":"ANY STRING IS OK"}') { [200, {}, ''] }
end
connection.post('/foo', JSON.dump(name: 'YK', created_at: Time.now))
stubs.verify_stubbed_calls
```
In this case, it's difficult to make tests always pass with
`"created_at"` because the value is dynamic. So, I came up with an idea
to pass a proc as a stubbed value and compare bodies, inside the proc:
```ruby
stubs = Faraday::Adapter::Test::Stubs.new do |stub|
check = -> (request_body) { JSON.parse(request_body).slice('name') == { 'name' => 'YK' } }
stub.post('/foo', check) { [200, {}, ''] }
end
connection.post('/foo', JSON.dump(name: 'YK', created_at: Time.now))
stubs.verify_stubbed_calls
```
I believe this would be flexible but compatible with the previous behavior.
