fix: Correctly canonicalize query param spaces

[lionel-rowe]

This, along with lucacasonato/deno_s3#55, will fix lucacasonato/deno_s3#54.

URLSearchParams#toString() canonicalizes spaces in keys or values to +:

new URL('https://example.com/?a%20b=c%20d').searchParams.toString() // 'a+b=c+d'

Whereas AWS requires them to be percent-encoded. Per https://docs.aws.amazon.com/general/latest/gr/create-signed-request.html#create-canonical-request:

CanonicalQueryString – The URL-encoded query string parameters, separated by ampersands (&). Percent-encode reserved characters, including the space character.

[benatkin]

I'm interested in this library, and I think I would need this to be able to use it.

I saw there's a test in the s3 PR, perhaps one could be added here as well?