fix(positioning) don't modify readonly value

[cmpilato]

(Positioning.position): element.getBoundingClientRect() returns a read-only value, so don't assign it directly to a variable whose properties are subsequently modified.

[codecov]

Codecov Report

Merging #2042 into development will decrease coverage by 0.03%.
The diff coverage is 0%.

@@               Coverage Diff               @@
##           development    #2042      +/-   ##
===============================================
- Coverage        87.16%   87.12%   -0.04%     
===============================================
  Files               85       85              
  Lines             2259     2260       +1     
  Branches           292      292              
===============================================
  Hits              1969     1969              
- Misses             187      188       +1     
  Partials           103      103

Impacted Files	Coverage Δ
src/positioning/ng-positioning.ts	67.41% <0%> (-0.77%)	⬇️

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[cmpilato]

So ... if I understand the codecov report correctly, because I took code that had no coverage and made it have more lines, I've made things worse. Fair enough.

[valorkin]

Thanks for PR, didn't know that it is readonly!