More validation of HTTP/1.1 messages

More validation of HTTP/1.1 messages. (#321)

Add validation for the following, according to the ABNF defined in RFC-7230.
- header field-names
- header field-values
- request method
- request-path aka URI
- response reason-phrase

Previously, we did an OK job validating incoming messages, but did no validation whatsoever of outgoing messages.

Caveats:
- URI validation is extremely basic for now.

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