Fix id of anyof select

[epicfaace]

This fixes a change that was done in #1118. It seems like the id should be suffixed with a double underscore, not a single underscore.

For example, see examples here (https://github.com/mozilla-services/react-jsonschema-form/blob/master/src/components/fields/ArrayField.js#L27)

[epicfaace]

@LucianBuzzo could you please review this?

[edi9999]

Can you add a test to make sure this does not regress ?

[epicfaace]

@edi9999 I just changed the id, so I'm not sure how a test could meaningfully cover this change; what kind of test do you have in mind?

[edi9999]

I mean a test like here : https://github.com/mozilla-services/react-jsonschema-form/blob/master/test/BooleanField_test.js#L40

Where you render that component and verify that the id is the expected one.

[LucianBuzzo]

This looks good to me, can you please add a test that checks that the ID is correct?

[epicfaace]

I've added tests -- but I realized that both the oneof and anyof widgets have the same id (i.e., "__anyof_select" instead of "__oneof_select" and "__anyof_select"). Was this intentional or do you think we should fix this as well?

[edi9999]

Good catch, I think it is not intentional and we should fix it.

[epicfaace]

This is technically a backwards incompatible change anyway, so let's just add this in v2.

[epicfaace]

@LucianBuzzo / @edi9999 could you review?