Fix: Resolve referenced schema in oneOf combination

[localheinz]

This PR

• assert that a referenced schema in a oneOf combination is resolved
• resolves referenced schemas in oneOf combinations

Follows #45.

[codecov]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #47   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity      104    104           
=======================================
  Files            13     13           
  Lines           290    292    +2     
=======================================
+ Hits            290    292    +2

Impacted Files	Coverage Δ	Complexity Δ
src/SchemaNormalizer.php	100% <100%> (ø)	38 <0> (ø)	⬇️

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