Bugfix: Prevent double-prefixing when using combineActions

[SuaYoo]

Using the prefix option in conjunction with combineActions will prefix the combined action type again. For example:

const options = { prefix: 'my-prefix' };
const { action1, action2 } = createActions('ACTION_1', 'ACTION_2', options)

const combinedActionType = combineActions(action1, action2).toString()

console.log(combinedActionType)
// "my-prefix/my-prefix/ACTION_1||my-prefix/ACTION_2"

This PR fixes the issue by checking if the action type string already starts with the prefix + namespace.

[codecov]

Codecov Report

Merging #334 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #334   +/-   ##
=======================================
  Coverage   96.92%   96.92%           
=======================================
  Files          28       28           
  Lines         130      130           
  Branches       39       39           
=======================================
  Hits          126      126           
  Misses          3        3           
  Partials        1        1

Impacted Files	Coverage Δ
src/utils/flattenWhenNode.js	100% <100%> (ø)	⬆️

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[timche]

Sorry for the long waiting. Thank you very much 🙏 👏