Fix event handler binding issue

[eduardomourar]

Issue #, if available:

Description of changes:

While trying to access the instance with this in an event handler method, the following error was being thrown:

TypeError: Cannot read property 'typeName' of undefined

The change here fix the issue by binding the instance to the method.

[codecov]

Codecov Report

Merging #7 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
+ Coverage   94.24%   94.26%   +0.02%     
==========================================
  Files          13       13              
  Lines         834      837       +3     
  Branches      123      124       +1     
==========================================
+ Hits          786      789       +3     
  Misses         48       48              

Flag	Coverage Δ
#unittests	94.26% <100.00%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
src/resource.ts	96.41% <100.00%> (+0.05%)	⬆️

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