feat(formly-field): Adds parser for keys that contain arrays

[philjolly]

What

This adds a deep assign function to formly-field that allows for keys that contain array indexes to properly create the array if it does not exist. It also adds a flag to formlyConfig.extras to enable this parser lowering the impact on existing installations.

Why

angular.$parse (used in the setter) does not differentiate between object and array when setting values. In the case where a key points to an array that has not already been set in the model it will create an object with a key of the index. This issue has been discussed many times in the angular project with the decision each time that they will not fix it. i.e. angular/angular.js#9850

How

In formly-field the function deepAssign has been added which recursively follows the path provided in the key establishing the proper object/array nodes if they do not exist, there is also a check function added to identify keys that contain array indexes. In formlyConfig.extras a flag called parseKeyArrays (defaulted to false) controls whether or not to utilize the deepAssign function in the setter so that existing implementations will not be affected.

For issue #706

Checklist:

• Follows the commit message conventions
• Is rebased with master
• Is only one (maybe two) commits

angular.$parse does not (and will not angular/angular.js#9850) properly
handle arrays in keys unless they have already been created in the model. This fix/feature adds a
separate parser for these circumstances and a formlyConfig.extras flag to control its use. The flag
is in place to minimize impact on current functionality of the setter.

This is in support of #706

[codecov-io]

Current coverage is 95.93% (diff: 100%)

Merging #709 into master will increase coverage by 0.04%

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[kentcdodds]

LGTM! I'll let someone from @formly-js/angular-formly-collaborators-read give the thumbs up and someone from @formly-js/angular-formly-collaborators merge the PR.

[gillchristian]

LGTM 👍

[kwypchlo]

Looks good. I would even suggest to include this as default behavior.

[kentcdodds]

Agreed. We could make that a breaking change in a future release.