Implement conditional requests: If-Match, If-Modified-Since, If-None-Match, If-Range, If-Unmodified-Since and Last-Modified

[nickl-]

This feature is dependent on the implementation of the ETag see #40

Implement capabilities required for conditional requests:

• If-Match,
• If-Modified-Since,
• If-None-Match,
• If-Range,
• If-Unmodified-Since and
• Last-Modified (Not exactly a conditional request but utilizes the same mechanisms)

Because these requests require knowledge of the actual response and to prevent us having to re-compose the response for every question this will benefit considerably from a server side cache solution as suggested in #60

Did I miss anything?

What are your thoughts, suggestions, special requirements? Do you know of any current implementations we might benefit from referencing? Warnings? Concerns? Are you able to help?

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