Mark FormData support as optional

[josh]

Hypothetically, if a browser doesn't not implement FormData but did implement fetch(), how should Response.prototype.formData behave? Should Response.prototype.formData be undefined or should calling Response.prototype.formData throw some sort of error?

Chrome Canary currently does not implement Response.prototype.formData for an example. Also, old polyfilled IEs won't have FormData.

/cc @dgraham @mislav @annevk

[dgraham]

I wrote the FormData support for completeness with the spec. We actually have no use cases for it on github.com.

I guess I don't understand the hypothetical situation described. All browsers with fetch also have FormData. Is this a question specifically for IE 9?

[josh]

The same applies to blob. PhantomJS doesn't implement Blob() so it skips defining Response.prototype.blob.

[dgraham]

Ah, PhantomJS is the new IE 9.

[josh]

After this PR, all supported tests pass in Chrome Canary.

[annevk]

If there's no support I would expect the property to not be there so you can feature test it.