fix: should not set "Content-Encoding" for a direct request of a compressed file

[hi-ogawa]

• closes Downloading gzipped files in browsers #158

Content-Encoding should be used only when sirv is serving pre-compressed files (e.g. request: test.csv --> response: test.csv.gz), but it should be avoided when users are requesting a compressed file directly (e.g. test.csv.gz).

Citing from https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Encoding

If the original media is encoded in some way (e.g. a zip file) then this information would not be included in the Content-Encoding header.

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The summary of changes in this PR is:

• replace extns: string with extns: { ext: string, encoded: boolean }[],
• when matching a file with viaLocal or viaCache, it also returns { name: string, encoded: boolean }
• move Content-Type and Content-Encoding handling from toHeaders to send since it now requires name and encoded to decide these two headers.

Diff looks a little involved, but I hope this is clear and correct. I would appreciate a review. Thanks!