Adds Last-Modified with date from mbtiles

[buma]

I added Last-Modified field to headers. Value is last modified time of each mbtiles file.

Code probably isn't great since this is my first Go code.

[brendan-ward]

@buma first off, thank you for making a foray into Go to help with this project! A few minor fixes, and I think this will be a nice addition.

Note: semicolons at the end of lines are unnecessary, and are automatically stripped by gofmt (please make sure to run that before committing Go code...)

I'd like to see modTime excluded from the tilejson response body; it isn't part of the specification and I didn't find a common convention for it. Add it to the list of excluded keys around line 407.

[brendan-ward]

Lines 155 - 167 could be done more simply like:

fileStat, err := os.Stat(filename)
if err != nil {
    log.Errorf("could not read file stats for mbtiles file: %s\n", filename)
    continue
}

then line 190 could be

metadata["modTime"] = fileStat.ModTime().Round(time.Second)

[buma]

I fixed, what you requested. Code is much nicer now.

[brendan-ward]

Thanks @buma 👍