Correctly reset the parser

[bryanp]

Resolves https://github.com/httprb/http/issues/685.

irb(main):004:0> HTTP.via('127.0.0.1', 8080).get('https://httpbin.org/get')
=> #<HTTP::Response/1.1 200 OK {"Date"=>"Thu, 09 Sep 2021 22:27:04 GMT", "Content-Type"=>"application/json", "Content-Length"=>"235", "Connection"=>"close", "Server"=>"gunicorn/19.9.0", "Access-Control-Allow-Origin"=>"*", "Access-Control-Allow-Credentials"=>"true"}>

I propose we include this as part of a 0.5.2 release.

[tarcieri]

Thank you!