Exceptions are raised on header retrieval for invalid header lines

[weierophinney]

Per a report I've received recently, calling getHeader() can result in an exception for malformed header lines, leading to unexpected behavior.

As an example, consider the following:

GET: /some/path
HOST: domain.org
Useragent: h4ckerbot

If a user then attempts to pull the User-Agent header as follows:

$request->getHeader('User-Agent'); // or User_Agent, useragent, etc.

an exception will be thrown, as the header line fails validation (the UserAgent header class expects a line matching /^user-agent: .*?$/i).

The expectation is that this would instead return boolean false (header is not defined and/or malformed), or whatever default value is provided as a second argument to getHeader().

In analyzing the interactions between Request::getHeader(), Headers::has()/Headers::get(), and PhpEnviroment\Request::marshalHeaders(), I think the best approach is to modify Headers::has() to perform the logic necessary to lazy-load a header if no instance yet exists for it, wrapping that logic in a try/catch block, and returning boolean false if an exception is caught.

The rationale is because Request::getHeaders() performs a call to Headers::has(), and only calls Headers::get() if the has() returns true, returning $default if it does not. Making this change would allow Request::getHeader() to return $default in such cases, which is what users expect.