Format the Last-Modified timestamp strictly according to the HTTP
Specification.

PHP's DATE_RFC1123 does not generate correct timestamps when the server is
not operating on GMT.
Merge remote-tracking branch 'solarissmoke/last-modified-bug' into release-2.3.1
Let Resource Locator store and return Closure values
Let legacy View::setData not invoke stored Closure values
Revise how we store Closure values in Set
Add contributing file
Update README
Format Expires header according to HTTP specification.
Merge remote-tracking branch 'solarissmoke/last-modified-bug' into release-2.3.1
Missing __isset and __unset
__unset
Pretty exceptions only in debug
Updated the Install docs URL
Modify tests for Last-Modified and Expires headers to enforce RFC compliance.
Merge pull request #569 from solarissmoke/http-header-tests

Modify tests for Last-Modified and Expires headers to enforce RFC compliance
Fix issue #568: Parse string 'expires' before encoding secure cookie.

The `\Slim\Http\Util::encodeSecureCookie` function requires the
expires parameter to be a int. A string typed 'expires' or
'cookie.lifetime' would pass a string in this case, which causes the
response cookie to be broken when parsed by `decodeSecureCookie`.
Adds a unit test to serialize and decrypt cookies with string expires.

This test complements the pull request #571 (related to issue #568).
Merge branch 'release-2.3.1'