Update TokenParser Function

[kristinapathak]

bascule/acquire/bearer.go

Lines 136 to 144 in c011b12

	respBody, errRead := ioutil.ReadAll(resp.Body)
	if errRead != nil {
	return "", fmt.Errorf("error reading HTTP response body: %v", errRead)
	}
	token, err := acquirer.options.GetToken(respBody)
	if err != nil {
	return "", fmt.Errorf("error parsing bearer token from http response body: %v", err)
	}

Currently, we don't check the response's Content-Type header before reading the body and passing it to the TokenParser. It would be nice to provide this information to the TokenParser, either as a string parameter or by passing the http.Request and allowing the TokenParser to do its own validation and parsing from there. Either option allows us to give up on parsing something if its not in the form we expect, and allows for more flexible parsing - a TokenParser could parse differently depending on the Content-Type provided.

Passing the full response would make it the TokenParser's responsibility to read and close the response body. Passing the Content-Type as a string allows TokenParsers to have no dependency on the http package.