fix: handle invalid dates properly

[flakey5]

Closes #262

[targos]

Maybe I'm missing something, but the fix doesn't look right. At least in Node.js, an invalid date is still an instance of Date.

[MattIPv4]

As @targos noted, checking if a date is an instance of Date does not tell you whether its invalid or not -- you need check using the global isNaN.

isNaN(new Date()); // false
isNaN(new Date("aaa")); // true

[MattIPv4]

Could a test or two be added for these headers when they're set to valid + invalid values in requests?

[ovflowd]

Could a test or two be added for these headers when they're set to valid + invalid values in requests?

cc @flakey5

[ovflowd]

Awesome stuff!