chore: improve code qualities and use idiomatic way to implementation

[nekomeowww]

Description

Improvements of #18

• Removed unnecessary code
• Use http.StatusXxx instead of hardcoded numbers when checking http status
• Replaced incorrect use of fmt.Printf(..) with log.Printf(...) with idiomatic prefixes [WARN]
• Replaced incorrect use and deprecated ioutil.ReadAll(...) with *bytes.Buffer and io.Copy(...) for better performance

[tmc]

LGTM