fix(#210): Provide a method to return all headers (not only first header) of a given key

[germanhs]

This is the fix for #210

It provides an additional method for the ClientResponse in response.go to retrieve all headers of a given key.

The current implementation only allows to retrieve the first header.

If you don't want to change the interface I would suggest to provide an extended version to which we can cast the Client Response.

[germanhs]

In the test structures the GetHeaders function was missing. Can you rerun the workflow please? :)
Edit: I also added a small unit test.

[youyuanwu]

Should ClientResponse have a method to expose http.Response? If someone else want to handle special cases in the ClientResponse, the interface needs to be changed again.

@casualjim I will let you decide. This code is ready to merge.

[leason00]

@germanhs
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