Simplify Crawly.fetch/2 function

[oltarasenko]

I decided to refactor this code a bit, so it is more simple (and also credo was complaining about it). Hopefully Ziink will forgive me :)

[github-actions]

Tests summary

Finished in 11.7 seconds (0.7s async, 11.0s sync)
Randomized with seed 881356

✅ 0 failures (135 tests)
✅ 76.36% coverage (75% is the minimum)

Coverage details

Percentage | Module
-----------|--------------------------
     0.00% | Crawly.Fetchers.Splash
     0.00% | Crawly.Loggers.SendToUiBackend
     0.00% | Crawly.Response
     0.00% | Mix.Tasks.Crawly.Gen.Config
     0.00% | String.Chars.Crawly.Request
     0.00% | UtilsTestSpider
    40.00% | PipelineTestSpider
    42.86% | TestSpider
    53.85% | Mix.Tasks.Crawly.Gen.Spider
    60.67% | Crawly.API.Router
    61.54% | Crawly.Models.YMLSpider
    66.67% | Crawly.Models.Job
    68.42% | Crawly
    75.00% | Crawly.Pipelines.JSONEncoder
    76.47% | Crawly.SimpleStorage
    76.92% | Crawly.Fetchers.CrawlyRenderServer
    77.78% | Crawly.Pipelines.Experimental.SendToUI
    80.00% | Crawly.EngineSup
    82.61% | Crawly.Pipelines.WriteToFile
    83.33% | Crawly.Middlewares.UserAgent
    84.54% | Crawly.Utils
    86.84% | Crawly.Engine
    87.50% | Crawly.Application
    87.50% | TestUtils
    88.37% | Crawly.RequestsStorage
    88.46% | Crawly.RequestsStorage.Worker
    89.06% | Crawly.Manager
    90.00% | Crawly.Pipelines.Validate
    91.84% | Crawly.Worker
    92.11% | Crawly.DataStorage
    92.86% | Crawly.DataStorage.Worker
   100.00% | Crawly.Fetchers.Fetcher
   100.00% | Crawly.Fetchers.HTTPoisonFetcher
   100.00% | Crawly.ManagerSup
   100.00% | Crawly.Middlewares.AutoCookiesManager
   100.00% | Crawly.Middlewares.DomainFilter
   100.00% | Crawly.Middlewares.RequestOptions
   100.00% | Crawly.Middlewares.RobotsTxt
   100.00% | Crawly.Middlewares.SameDomainFilter
   100.00% | Crawly.Middlewares.UniqueRequest
   100.00% | Crawly.ParsedItem
   100.00% | Crawly.Pipeline
   100.00% | Crawly.Pipelines.CSVEncoder
   100.00% | Crawly.Pipelines.DuplicatesFilter
   100.00% | Crawly.Request
   100.00% | Crawly.Settings
   100.00% | Crawly.Spider
-----------|--------------------------
    76.36% | Total