Fix encoder naming convention and restore JPEG redirect test

[Copilot]

Addresses review feedback on PR #65 regarding naming consistency and test coverage.

Changes

• Renamed encodeWEBP → encodeWebP to match existing encoder naming pattern (encodeJPEG, encodeGIF, encodePNG)
• Updated extencoders map to reference corrected function name
• Restored JPEG redirect test case verifying .jpeg → .jpg redirect behavior (HTTP 301)

// Before
func encodeWEBP(out io.Writer, in image.Image) error {
    return nativewebp.Encode(out, in, nil)
}

// After
func encodeWebP(out io.Writer, in image.Image) error {
    return nativewebp.Encode(out, in, nil)
}

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