Aaccording to the docs, if reqHeaders is not specified, it will skip them.
If no request headers are specified for mocking then Nock will
automatically skip matching of request headers
In case you're validating other headers, and if x-md5-checksum is present, but you don't know it's value, you can use a function or a regex to validate any value, or just a valid md5
nock('http://www.example-com', {
reqHeaders: {
'x-md5-checksum': /[a-fA-F0-9]{32}/
// or
'x-md5-checksum': value => true // I don't care about the value
}
})
// or use .matchHeader
.matchHeader('x-md5-checksum', value => true)
.get('/')
.reply(200)