How to do a https request with bad certificate?

Proper way (as of Go 1.13) (provided by answer below):

customTransport := http.DefaultTransport.(*http.Transport).Clone()
customTransport.TLSClientConfig = &tls.Config{InsecureSkipVerify: true}
client := &http.Client{Transport: customTransport}

Original Answer:

Here's a way to do it without losing the default settings of the DefaultTransport, and without needing the fake request as per user comment.

defaultTransport := http.DefaultTransport.(*http.Transport)

// Create new Transport that ignores self-signed SSL
customTransport := &http.Transport{
  Proxy:                 defaultTransport.Proxy,
  DialContext:           defaultTransport.DialContext,
  MaxIdleConns:          defaultTransport.MaxIdleConns,
  IdleConnTimeout:       defaultTransport.IdleConnTimeout,
  ExpectContinueTimeout: defaultTransport.ExpectContinueTimeout,
  TLSHandshakeTimeout:   defaultTransport.TLSHandshakeTimeout,
  TLSClientConfig:       &tls.Config{InsecureSkipVerify: true},
}
client := &http.Client{Transport: customTransport}

Shorter way:

customTransport := &(*http.DefaultTransport.(*http.Transport)) // make shallow copy
customTransport.TLSClientConfig = &tls.Config{InsecureSkipVerify: true}
client := &http.Client{Transport: customTransport}

Warning: For testing/development purposes only. Anything else, proceed at your own risk!!!

Security note: Disabling security checks is dangerous and should be avoided

You can disable security checks globally for all requests of the default client:

package main

import (
    "fmt"
    "net/http"
    "crypto/tls"
)

func main() {
    http.DefaultTransport.(*http.Transport).TLSClientConfig = &tls.Config{InsecureSkipVerify: true}
    _, err := http.Get("https://golang.org/")
    if err != nil {
        fmt.Println(err)
    }
}

You can disable security check for a client:

package main

import (
    "fmt"
    "net/http"
    "crypto/tls"
)

func main() {
    tr := &http.Transport{
        TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
    }
    client := &http.Client{Transport: tr}
    _, err := client.Get("https://golang.org/")
    if err != nil {
        fmt.Println(err)
    }
}