PLT-1649: add response_url support for custom slash commands (#6739)

* add response_url support for custom slash commands

* pr suggestions

* pr update / suggestion

* test fix

model/command_response.go

@@ -6,6 +6,7 @@ package model
 import (
 	"encoding/json"
 	"io"
+	"io/ioutil"
 )
 
 const (
@@ -31,6 +32,22 @@ func (o *CommandResponse) ToJson() string {
 	}
 }
 
+func CommandResponseFromHTTPBody(contentType string, body io.Reader) *CommandResponse {
+	if contentType == "application/json" {
+		return CommandResponseFromJson(body)
+	}
+	if b, err := ioutil.ReadAll(body); err == nil {
+		return CommandResponseFromPlainText(string(b))
+	}
+	return nil
+}
+
+func CommandResponseFromPlainText(text string) *CommandResponse {
+	return &CommandResponse{
+		Text: text,
+	}
+}
+
 func CommandResponseFromJson(data io.Reader) *CommandResponse {
 	decoder := json.NewDecoder(data)
 	var o CommandResponse