Make StaticRoute support Last-Modified and If-Modified-Since headers

[magv]

This change makes StaticRoute add Last-Modified headers to the responses and recognize If-Modified-Since headers in the requests. If If-Modified-Since header is present and is equal to the modification time of the file being served, StaticRoute will now respond with a 304 "Not Modified" response.

This is a small feature, but rather useful if you hit "Refresh" button often.

[asvetlov]

Please split the test into several (five?) ones.

[asvetlov]

The PR is good but I guess to add if_modified_since readonly property to Request and last_modified read-write property to StreamResponse (like content_length for example).

[asvetlov]

See also #165

[magv]

I've split the tests somewhat (some of them are superfluous though).

Would you like the if_modified_since property to be a timestamp or a string? I think such a property would make sense if it's value was the parsed timestamp, but my proposed code doesn't really parse the If-Modified-Since header, it just compares it char-by-char with the mtime string -- this is faster, simpler, and just as useful (because user agents usually just copy whatever you sent in the Last-Modified header without interpreting it); this behaviour also protects against the situation where your clock was rewound backwards, and the client has a timestamp far into the future.

[asvetlov]

Yes, properties make sense only if they have datetime type (timezone aware BTW).
I think string parsing time is vanishing tiny comparing to overall response time.

By standard it is timestamp. If you need arbitrary data like hash please use ETAG header.

[magv]

Something like this then? I think it's overengineering, to be honest.

[asvetlov]

Move it into Request object: it's request-only header

[asvetlov]

I want to make properties for all common HTTP headers eventually.
if_modified_since and last_modified are not necessary for static file handling task but I like to have those as well as other absent fields.

[asvetlov]

Thanks!

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