Enforce restrictions on request-target formats

[natemcmaster]

We currently accept some malformed requests that are not adherent to the HTTP/1.1 spec.

Example:

• relative paths GET ../../ HTTP/1.1 is not a valid format at all. Should be rejected.
• authority-form GET www.contoso.com HTTP/1.1 is invalid, but Kestrel will accept anyways.
• asterisk-form GET * HTTP/1.1 is invalid, but Kestrel will accept anyways.

Restrictions on request-target that are not currently enforced:

• authority-form are only valid for CONNECT requests https://tools.ietf.org/html/rfc7230#section-5.3.3
• asterisk-form should only by used with OPTIONS requests https://tools.ietf.org/html/rfc7230#section-5.3.4