go 1.13 net/http not support Header.Values(name)

[leason00]

func (Header) Values ¶
added in go1.14
func (h Header) Values(key string) []string
Values returns all values associated with the given key. It is case insensitive; textproto.CanonicalMIMEHeaderKey is used to canonicalize the provided key. To use non-canonical keys, access the map directly. The returned slice is not a copy.

https://pkg.go.dev/net/http#Header

but in this project:
go 1.13 in go.mod
use 1.14 featur:
func (r response) GetHeaders(name string) []string {
return r.resp.Header.Values(name)
}
This PR:#211 introduces this problem.

[youyuanwu]

Bumped version to go 1.15 in go.mod.