Header option bugs

[arthurfranca]

Middleware localOptions.headers currently are ignored (only globalOptions.headers are used)

apicache/src/apicache.js

Lines 189 to 195 in f27cb2b

	Object.keys(globalOptions.headers).forEach(function(name) {
	res.setHeader(name, globalOptions.headers[name])
	})
	res.writeHead = function() {
	// add cache control headers
	if (!globalOptions.headers['cache-control']) {

Also, following code is ignoring the possibility of cacheObject.headers['cache-control'] being a value previously overrided by global/localOptions, like e.g. max-age=0, must-revalidate or no-cache or maybe a max-age lower than that being currently set.
I mean, if there was global/localOptions.headers['cache-control'] that was used when caching response, i think it should be used when sending cached response

apicache/src/apicache.js

Lines 251 to 259 in f27cb2b

	Object.assign(headers, filterBlacklistedHeaders(cacheObject.headers || {}), {
	// set properly-decremented max-age header. This ensures that max-age is in sync with the cache expiration.
	'cache-control':
	'max-age=' +
	Math.max(
	0,
	(duration / 1000 - (new Date().getTime() / 1000 - cacheObject.timestamp)).toFixed(0)
	),
	})

[stjosh]

I think #163 tries to fix the same issue with not counting that 'cache-control' could already be set... This is really bugging me as well, as I do invalidate my cache on every write to the DB and don't want my clients to sit around with stale responses...

@kwhitley Any chance this could be fixed soon? I had to disable my cache again due to not being able to override the cache-control header...