[6.x] Fix last_modified option in SetCacheHeader

[aponscat]

Fix problem with last_modified option

The SetCacheHeaders Middleware accept a parameter last_modified, the problem is that the middleware passes only strings as parameters and the setCache needs a DateTime compatible object.

This fix parses the string passed in the last_modified option in two ways:

1. if the parameter is a number, a unix timestamp is assumed, parsed with Carbon and replaced in the options array
2. if the parameter is a string, it's send to the Carbon parse function in order to create a date and then is replaced in the options array

Two new tests created

1. testLastModifiedUnixTime

• Gets the current unix timestamp, passes to the last_modified option of the CacheHeaders Middleware and checks that the response contains the Last-Modified header with that exact date.

2. testLastModifiedStringDate

• Passes my birthdate to the last_modified option of the CacheHeaders Middleware and checks that the response contains the Last-Modified header with that exact date.

[driesvints]

Hey @aponscat. Almost there. Just a few styling issues left: https://github.styleci.io/analyses/zEgQeD?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

[aponscat]

Sent a new commit

[aponscat]

Hope that now it's ok.

Sorry for the first low quality pull requests.

And thanks again to you and Taylor for your great job!