Override Last-Modified header in web.FileResponse

[ykvch]

🐣 Is your feature request related to a problem? Please describe.
web.FileResponse currently does not allow overriding "Last-Modified" header via last_modified attribute (while parent web.StreamResponse allows it).
FileResponse instead always sets actual file mtime even if last_modified attribute has been set before.

💡 Describe the solution you'd like
Change line: https://github.com/aio-libs/aiohttp/blob/master/aiohttp/web_fileresponse.py#L148 to:

    if self.last_modified is None:
        self.last_modified = st.st_mtime

So that self.last_modified does not get reset by prepare() method if it has been set explicitly.

❓ Describe alternatives you've considered
Also considered setting header directly via headers={"Last-Modified": ...} but it doesn't work (obviously it is not the correct way).

📋 Additional context

In order to also mimic "Last-Modified" removal (like in web.StreamResponse) extra steps are required:

• Insert empty last_modified header in init: self._headers[hdrs.LAST_MODIFIED] = None
• Extend check to see if it has been explicitly removed (same line 148):

    if self.last_modified is None and hdrs.LAST_MODIFIED in self._headers:
        self.last_modified = st.st_mtime