GenerateSW does not support method option in runtimeCaching

[chrisdns]

Library Affected:
workbox-build, workbox-webpack-plugin

Browser & Platform:
Google Chrome Version 69.0.3497.92 (Official Build) (64-bit)

Issue or Feature Request Description:
I would like to use POST method in runtimeCaching options from WorkboxPlugin.GenerateSW({ }) plugin instead of GET.

It is not possible at the moment to change the default method ; it leads to a validationError

ValidationError: child "runtimeCaching" fails because ["runtimeCaching" at position 0 fails because ["method" is not a supported parameter.]].

It should be possible to change default method, I can make a PR if you think this is relevant.

[jeffposnick]

A PR would be appreciated—thanks!

This was just an oversight; there's already logic in place to read in the method, but the validation logic doesn't allow for it to be set, so I think that validation logic is the only thing that would need to change.