Update computedFn warning to respect mobx global computedRequiresReaction

[wemyss]

Purpose

Addresses this issue: #268

This will allow consumers to test code wrapped with computedFn easily by using

mobx.configure({ computedRequiresReaction: false })

Currently in order to test a computed funciton you either:

1. Ignore the warnings, e.g. --silent in jest
2. Wrap the tested function in an autorun in each test or during setup before each test.

Notes

There will be a behavioural change in when a computedFn warning is triggered as the default for computedRequiresReaction is false. This will make it consistent in behaviour with the base computed, but if it is desirable to maintain the existing behavior we could add another option... I'm not sure where that would live, as mobx-utils has no global state and it seems odd to put computedFnRequiresReaction within the mobx global state - thoughts?

[ezze]

@mweststrate @urugator Can this be merged, guys?

[urugator]

Sorry for the late response, but it should probably also respect requiresReaction flag from the options, taking precendece over global config, like here:
https://github.com/mobxjs/mobx/blob/27efa3cc637e3195589874990c23d4de82c12072/packages/mobx/src/core/computedvalue.ts#L311-L319

[NaridaL]

Included in v6.0.7

[NaridaL]

@urugator ah sorry, I've already published it :/

[urugator]

No problem, it's good you keep things moving forward here.