Can not produce Signature for Responses

[aljones15]

Not sure if this library was intended to, but

http-signature-header/lib/index.js

Lines 218 to 223 in 2070a68

	api.createSignatureString = ({includeHeaders, requestOptions} = {}) => {
	assert.object(requestOptions, 'requestOptions');
	assert.object(requestOptions.headers, 'requestOption.headers');
	assert.string(requestOptions.method, 'requestOption.method');
	assert.string(requestOptions.url, 'requestOption.url');
	assert.optionalArrayOfString(includeHeaders, 'headers');

currently createSignatureString needs a method argument passed in.
Response HTTP Messages do not specify a method so
we either need to re-use the request's method (if available) or
allow createSignatureString to operate with out one. We could do both.
Basically we are not handling the case where a response message is signed with
(request-target) in the Signature headers parameter.