Add spec for Client.request
Update HTTPkit.run/stop specs
Add spec for HTTPkit.sleep
Turn #add_extra_headers into MandatoryHandler
Append "Handler" to handler class names
Update Flay config
Prepare handler invocation for immutable requests/responses
Update Flay config
Fix Client handlers being optional
Update Flay config
Update Client#initialize specs
Kill mutations in Client
Kill more Client and Server mutations
Run Mutant in CI, but ignore result
Fix CI command
Update mutation coverage
Update Flay config
Update README
Make Serializer#serialize block
Add specs for Serializer