Bugfix/event listener callback issue

[bartoszglow]

Closes #100
Closes #91

This PR:

• makes the data layer always behave the same: it does not matter whether the custom code is executed before or after the data layer script has loaded
• makes the callback function fire just once, whether registering an event listener before or after adobe-client-data-layer.min.js has loaded
• adds tests to verify the behavior
• adds tests to verify nested anonymous functions behaviour

Examples:

1. Event listener with scope set to past, late ACDL initialization:

  adobeDataLayer.push(function(adobeDataLayer) {
    adobeDataLayer.addEventListener('change', function(event) { ... }, { scope: 'past' });
  });

  adobeDataLayer.push({
    event: 'change'
  });

  // ACDL gets initialized afterwards

Result: event listener will not be triggered

2. Event listener with scope set to future, late ACDL initialization:

  adobeDataLayer.push(function(adobeDataLayer) {
    adobeDataLayer.addEventListener('change', function(event) { ... }, { scope: 'future' });
  });

  adobeDataLayer.push({
    event: 'change'
  });

  // ACDL gets initialized afterwards

Result: event listener will be triggered

3. Event listener with scope set to past, early ACDL initialization:

  // ACDL gets initialized before

  adobeDataLayer.push(function(adobeDataLayer) {
    adobeDataLayer.addEventListener('change', function(event) { ... }, { scope: 'past' });
  });

  adobeDataLayer.push({
    event: 'change'
  });

Result: event listener will not be triggered

4. Event listener with scope set to future, late ACDL initialization:

  // ACDL gets initialized before

  adobeDataLayer.push(function(adobeDataLayer) {
    adobeDataLayer.addEventListener('change', function(event) { ... }, { scope: 'future' });
  });

  adobeDataLayer.push({
    event: 'change'
  });

Result: event listener will be triggered

[jckautzmann]

We should have new tests to verify the correct behaviour as described in #100:

1. add event listener before DL initialization -> add event -> verify that the listener is triggered as soon as the event is processed
2. add event listener after DL initialization -> add event -> verify that the listener is triggered right away

[codecov]

Codecov Report

Merging #101 into master will decrease coverage by 0.67%.
The diff coverage is 91.66%.

@@             Coverage Diff             @@
##            master     #101      +/-   ##
===========================================
- Coverage   100.00%   99.32%   -0.68%     
===========================================
  Files           12       11       -1     
  Lines          302      295       -7     
  Branches        69       66       -3     
===========================================
- Hits           302      293       -9     
- Misses           0        2       +2     

Impacted Files	Coverage Δ
src/listenerManager.js	100.00% <ø> (ø)
src/dataLayerManager.js	98.27% <90.90%> (-1.73%)	⬇️
src/utils/dataMatchesContraints.js	100.00% <100.00%> (ø)

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