fix endpoint path removal

[srchase]

Resolves #1730

Removes leading slash of relative path when resolving URIs. This preserves the path when a custom endpoint with a path is specified for a client.

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[codecov-io]

Codecov Report

Merging #1795 into master will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master    #1795      +/-   ##
============================================
- Coverage     94.19%   94.17%   -0.02%     
- Complexity     3070     3072       +2     
============================================
  Files           180      180              
  Lines          8210     8213       +3     
============================================
+ Hits           7733     7735       +2     
- Misses          477      478       +1

Impacted Files	Coverage Δ	Complexity Δ
src/Api/Serializer/RestSerializer.php	95.04% <66.66%> (-0.87%)	54 <0> (+2)

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