Not replay executionObservables for execute()

[ishkawa]

I found second call of execute() returns the same output which the first time returned.
This issue started to happen from version 2.1.0, and doesn't happen when one use input only.

This issue is caused by replaying execution observable. To fix this issue, this PR replaces sharedReplay(1) of executionObservable with share() so that execute() returns always return the latest execution observable.

I'm sorry for overlooking this issue.
I've also update unit tests to detect the same kind of issue.

[ishkawa]

I found another issue with execute(_:), which is related to #63.

When execute(_:) is called while action is executing, both the first observable and the second observable fails with notEnabled error.

• Expected sequence

  execute(_:):               `-a-b----`
  returned observable for a: `-----a--`
  returned observable for b: `---x----` (x: notEnabled)
  

• Actual sequence

  execute(_:):               `-a-b----`
  returned observable for a: `---x----` (x: notEnabled)
  returned observable for b: `---x----` (x: notEnabled)
  

I've fixed this issue and added test cases in f15eebb 🙇

[ashfurrow]

Great work, thanks!