Add support for OWS in header-fields

[Davoodeh]

https://www.rfc-editor.org/rfc/rfc7230#section-3.2

Each header field consists of a case-insensitive field name followed by a colon (":"), optional leading whitespace, the field value, and optional trailing whitespace.

    header-field   = field-name ":" OWS field-value OWS

Before this, the server would crash if the "optional" whitespace was missing.