Fix for creating ImageLayer with a string

[darrynten]

Image creation from a string was not working.

[codecov]

Codecov Report

Merging #26 into dev will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                dev      #26      +/-   ##
============================================
- Coverage     85.76%   85.73%   -0.03%     
  Complexity      201      201              
============================================
  Files            24       24              
  Lines           562      561       -1     
============================================
- Hits            482      481       -1     
  Misses           80       80

Impacted Files	Coverage Δ	Complexity Δ
src/Layers/Layer/ImageLayer.php	100% <100%> (ø)	6 <0> (ø)	⬇️

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