fix(docs): define mirror as default extend param

README.md

@@ -563,7 +563,7 @@ Image measures are always in pixels, unless otherwise indicated.
 - **url**         `string` - Fetch the image from a remote HTTP server. In order to use this you must pass the `-enable-url-source` flag.
 - **colorspace**  `string` - Use a custom color space for the output image. Allowed values are: `srgb` or `bw` (black&white)
 - **field**       `string` - Custom image form field name if using `multipart/form`. Defaults to: `file`
-- **extend**      `string` - Extend represents the image extend mode used when the edges of an image are extended. Defaults to `copy`. Allowed values are: `black`, `copy`, `mirror`, `white`, `lastpixel` and `background`. If `background` value is specified, you can define the desired extend RGB color via `background` param, such as `?extend=background&background=250,20,10`. For more info, see [libvips docs](http://www.vips.ecs.soton.ac.uk/supported/8.4/doc/html/libvips/libvips-conversion.html#VIPS-EXTEND-BACKGROUND:CAPS).
+- **extend**      `string` - Extend represents the image extend mode used when the edges of an image are extended. Defaults to `mirror`. Allowed values are: `black`, `copy`, `mirror`, `white`, `lastpixel` and `background`. If `background` value is specified, you can define the desired extend RGB color via `background` param, such as `?extend=background&background=250,20,10`. For more info, see [libvips docs](http://www.vips.ecs.soton.ac.uk/supported/8.4/doc/html/libvips/libvips-conversion.html#VIPS-EXTEND-BACKGROUND:CAPS).
 - **background**  `string` - Background RGB decimal base color to use when flattening transparent PNGs. Example: `255,200,150`
 - **sigma**       `float`  - Size of the gaussian mask to use when blurring an image. Example: `15.0`
 - **minampl**     `float`  - Minimum amplitude of the gaussian filter to use when blurring an image. Default: Example: `0.5`