add tests for components

[UzeyirMammadov]

field	tested populated	tested empty
schemas	✅	✅
responses	✅	❌
parameters	✅	❌
examples	✅	❌
requestBodies	✅	✅
headers	✅	✅
securitySchemes	✅	❌
links	✅	✅
callbacks	✅	✅
pathItems	❌	❌

we need tests for the cases with red cross (except pathItems cause parsing of those is not implemented yet I think).

[UzeyirMammadov]

is stuck until #74 is finished