[gatsby-plugin-netlify] default HTTP/2 push

[ghost]

Description

The Readme of "gatsby-plugin-netlify" states:

By default, the plugin will add HTTP/2 assets to server push the critical Gatsby scripts (ones that have the preload attribute already).

This does not reflect the current behaviour.

In src/constants.js the HTTP/2 push headers are disabled by default:
mergeLinkHeaders: false, // TODO: change this to true when gzip for server push is on netlify

I have a project on netlify with mergeLinkHeaders: true and did not experience any problems.